Chapter 20 Human Health and Disease by TEACHING CARE online tuition and coaching classes

Chapter 20 Human Health and Disease by TEACHING CARE online tuition and coaching classes


“Disease” is a very wide term. Any change from the normal state that causes discomfort or disability or impairs the health may be called a disease. The oxford English Dictionary defines disease as “a condition of the body or some part or organ of the body in which its functions are disturbed or deranged”. A person free of disease is often said to be healthy. This is not fully true. The term “Health” has a very wide scope. The World Health Organization (WHO) gave the following definition of health in 1948 –

“Health” is a state of complete physical, mental and social well-being, and not merely an absence of disease or infirmity”. The WHO definition of health recognizes three dimensions of health : physical mental and social. The physical health can be determined by various tests, but it is difficult to assess the mental health and social well- being.

 Disease Agents.

The disease agent is a factor (substance or force) which causes a disease by its excess or deficiency or absence. These agents are of five main types :

  • Biological Agents : These include viruses, rickettsias, bacteria, fungi, protozoans, helminthes and The biological agents are called pathogens (Gr. Pathos = disease; genes = producing).
  • Nutrient Agents : These comprise food components such as carbohydrates, fats, proteins, minerals, vitamins and
  • Chemical Agents : These are further of two types
  • Endogenous Chemical Agents : These are formed in the body itself and include hormones, enzymes, urea and uric
  • Exogenous Chemical Agents : These enter the body from outside by inhalation, ingestion or Pollutants (fumes, gases, dusts, metals) and allergens (spores, pollen) are examples.
  • Physical Agents : These include heat, cold, humidity, pressure radiation, electricity and
  • Mechanical Agents : These comprise chronic friction or other mechanical forces which result in injury, sprain, dislocation
  • Deficiency and Excess of substances : g. Hormones, enzymes.

Some diseases are caused by genetic disorders and lack or underdevelopment of organs. The agents for certain diseases such as peptic ulcers, coronary heart diseases and hypertension, are not fully known.

 Types of Diseases.

The diseases may be broadly classified into two types : Congenital and acquired.

(i) Congenital Diseases : These are anatomical or physiological abnormalities present from birth. They may be caused by (i) a single gene mutation (alkaptonuria, phenylketonuria, albinism, sickle-cell anaemia, haemophilia, colour blindness); (ii) chromosomal aberrations (Down’s syndrome, Klinefelter’s syndrome, Turner’s syndrome); or

  • environmental factors (cleft palate, harelip). Unlike the gene-and chromosome-induced congenital defects, environmentally caused abnormalities are not transmitted to the

(ii) Acquired Diseases : These diseases develop after birth. They are further of two types : communicable and non-communicable.




  • Communicable (Infectious) Diseases : These diseases are caused by viruses, rickettsias, bacteria, fungi, protozoans and
  • Noncommunicable (Noninfectious) Diseases : These diseases remain confined to the person who develops them and do not spread to The non-communicable diseases are of four kinds –
    • Organic or Degenerative Diseases : These diseases are due to malfunctioning of some of the important organs, g, heart diseases, epilepsy. Heart diseases result from the abnormal working of some part of this vital organ. Epilepsy may result from abnormal pressure on regions of the brain.
    • Deficiency Diseases : These diseases are produced by deficiency of nutrients, minerals, vitamins, and hormones, g., kwashiorkor, beriberi, goitre, diabetes are just a few from a long list.
    • Allergies : These diseases are caused when the body, which has become hypersensitive to certain foreign substance, comes in contact with that Hay fever is an allergic disease.
    • Cancer : This is caused by a uncontrolled growth of certain tissues in the body.

  Communicable Diseases.                                                                                                            

  • Meaning : The diseases which are caused by pathogens (viruses and living organisms) and readily spread from the infected to the healthy persons are called communicable or infectious diseases.

A German physician, Robert Koch, listed the following four conditions to establish that a specific pathogen causes a particular disease –

  • The suspected pathogen should be invariably present in the animals suffering from the disease and should not be found in healthy
  • The pathogens isolated from the diseased animal should be grown in a pure
  • When this culture is inoculated into a healthy host, the latter should develop the disease and show its characteristic
  • The pathogen should be recoverable from the experimental host, and it should be the same as the original


Kotch’s postulates proposed for animal diseases, hold good for human diseases also. However, his conditions

do not apply to viruses because they cannot be cultured on artificial media.

  • A communicable or Infectious Diseases : Caused by pathogens or biological They rapidly spread from one person to another and are of great concern of the society. They are further categorised as :
  • Viral diseases
  • Bacterial diseases
  • Protozoan diseases
  • Fungal diseases
  • Helminthes diseases
  • Sexually transmitted diseases (STD)
  • Diseases through blood transfusion
    • Control of Communicable Diseases : The Communicable diseases, beside simpering health, have been taking a heavy toll of human life in the Therefore, their control has always been the major problem of public health. Now, however, effective means of fighting these diseases have been found. This has not only greatly




reduced the toll taken by microbes, but has also increased man’s life expectancy. Efforts to control the communicable diseases have involved three major steps –

  • To know the nature of the disease, e., the causative agent and its life-history.
  • To find out mode of transmission of the disease, e., how the causative agents enter the human body.
  • To devise protective measures against the attack of the causative agent of the
    • Reservoir of Infection for Pathogens : Every pathogen has some reservoir where it normally lives when it is outside the host susceptible to the The reservoir varies for different pathogens. It may be soil, water, animals or other persons called carriers. The animals which act as reservoirs do not contract the diseases and are known as reservoir hosts.
    • Transmission of Diseases (Pathogens) : The diseases (pathogens) are transmitted from the reservoirs of infection to the healthy persons in the following ways –
  • Direct Transmission : The pathogens of some diseases reach the human body directly without intermediate This can occur as under –
    • Contact with Infected Persons : Certain diseases produce sores or lesions on the skin. Contact with materials discharged from these sores or lesions brings about infection. Ringworm, athlete’s foot, barber’s itch, chickenpox, smallpox, syphilis and gonorrhoea are spread by direct contact. Kissing also spreads infection. The diseases that are transmitted by direct contact are called contagious diseases.
    • Droplet Infection : Some diseases are caught by merely being in a confined place (room, theatre, bus) with an infected person. The latter throws out tiny droplets of mucus by coughing, sneezing, spitting or even talking. These droplets may contain pathogens (viruses, bacteria) dislodged from nasal membrane, throat, and lungs. Many of these droplets are inhaled. Diphtheria, scarlet fever, influenza, common cold, measles, mumps, tuberculosis, pneumonia, and whooping cough are spread by
    • Contact with Soil : The bacteria responsible for tetanus and blood poisoning enter the human body from the soil through Hence, skin injuries should not be neglected.
    • Animal Bites : Virus of rabies, or hydrophobia, is introduced through the wound caused by the bites of rabid animals, most commonly
    • Through Placenta : In the later part of pregnancy, due to age or injury, the placenta becomes permeable to certain pathogens such as virus of german measles and bacteria of The pathogens then pass from the maternal blood into the foetal blood.
  • Indirect Transmission : The pathogens of certain diseases reach the human body through some intermediate agents as explained below –
    • Arthropod Vectors : Insects transmit diseases in two different
    • Housefly carries the causative organisms of cholera, typhoid, dysentery and tuberculosis on the legs and mouth parts from faeces and sputum to food and drinks. The latter, if taken, cause infection. If also carries the microbes responsible for ophthalmia and conjunctivitis from eye to Ants, cockroaches and house crickets also carry disease germs to articles of food.
    • Certain blood-sucking insects carry disease-causing organisms in their body and transmit them with Human body-louse spreads typhus, rat flea transmits bubonic plague, tsetse fly spreads African sleeping sickness, sandfly transmits kala-azar and oriental sore, Aedes mosquito spreads yellow fever, Culex mosquito transmits filariasis, and Anopheles mosquito spreads malaria, ticks spread rocky mountain spotted fever.
    • Vehicle-borne Method : The causative organisms of dysentery, cholera and typhoid enter the human digestive tract with food, water and Most of the helminthes which produce diseases in man also get into the body in a similar way. Some diseases are transmitted through blood, e.g., AIDS.




  • Air-borne Method : The pathogens may reach the humans with air and dust. The epidemic typhus spreads by inhalation of dried faeces of infected
  • Fomite-borne Method : Many diseases are transmitted through the use of contaminated articles such as handkerchiefs, towels, clothes, utensils, toys, door handles, taps, soaps, syringes and surgical
  • Unclean Hands : The unclean hands may carry disease germs to food or mouth. Therefore, hands should be washed before taking
  • Human Carriers : Certain diseases, notably diphtheria and typhoid, are spread by human carriers. The latter are themselves healthy and immune, but have pathogenic organisms in their body. These pathogens are transmitted in the ways already
    • Classification of communicable Diseases : The communicable diseases are classified into seven types according to the nature of their causative
  • Viral Diseases : These are caused by They include chickenpox, smallpox, influenza, common cold, measles, mumps, polio, rabies, yellow fever, and sinus infections. The viruses are named after the disease they cause.
  • Rickettsial Diseases : These are caused by rickettsias, the obligate intracellular parasitic They include Rocky Mountain spotted fever, typh’s fever, trench fever and Q fever.
  • Bacterial Diseases : These are caused by bacteria. They include diphtheria, scarlet fever, tetanus, typhoid fever, tuberculosis, anthrax, cholera, food poisoning, and
  • Spirochaetal Diseases : These are caused by spirochaetes, the long, spiral, corkscrew-shaped bacteria. They cause
  • Protozoan Diseases : These are caused by protists. They include amoebic dysentery, malaria, kala-azar, oriental sore and sleeping
  • Fungal Diseases : These are caused by fungi, the non-green heterotrophic organisms. They include ringworm and athlete’s
  • Helminthes Diseases : These are caused by helminthes, i.e., flatworms and roundworms. They include liverrot, schistosomiasis, taeniasis and cysticercosis produced by flatworms; and ascariasis, enterobiasis, filariasis (elephantiasis), trichinosis, Guinea worm disease and hookworm disease caused by
    • How Pathogens Cause Diseases : Pathogens produce diseases in two ways : tissue damage and toxin
  • Tissue Damage : The bacteria responsible for tuberculosis damage cells and cause lesions in the lungs. Blood oozes from the lesions into the air sacs, leading to The bacteria that cause meningitis attack the protective membranes covering the brain. The virus of rabies destroys brain tissue. The polio virus damages motor nerve cells in the spinal cord.
  • Toxin Secretion : Many microbes produce powerful poisons, called toxins, which cause diseases. Toxins are of 2 types –
    • Exotoxins : These are released as soon as The diseases brought about by exotoxins include tetanus, scarlet fever, diphtheria, and botulism (food poisoning)
    • Endotoxins : These are retained in the bacterial cells and released when bacteria die and The diseases caused by endotoxins include typhoid fever, cholera, bubonic plague and dysentery.


 Non communicable diseases.

The main non-communicable diseases are diabetes, inflammatory diseases of joints such as arthritis, gout, cardiovascular diseases and cancer.




  • Diabetes Mellitus
  • Diabetes is characterised by chronic hyperglycemia which is excessive concentration of glucose in the
  • Diabetes is primarily a result of relative or complete lack of insulin secretion by the b cells of islets of Langerhans in
  • Diabetes is established by blood and urine sugar
  • Arthritis
  • Arthritis is any inflammatory condition of the joints characterised by pain and
  • Two kinds of arthritis are : rheumatoid arthritis and
  • There is no cure for arthritis; drugs are available which relieve
  • Rheumatoid arthritis is characterised by inflammation of the synovial
  • A kind of rheumatoid arthritis that occurs in younger people is Still’s
  • Osteoarthritis is a disease common among the elderly persons resulting from erosion of articular
  • Paraplegia refer to weakness or paralysis of both legs, often accompanied by loss of
  • Paraplegia is usually caused by a motor vehicle accident, sports accident, fall or gunshot
  • Gout
  • Gout results from accumulation of uric acid crystals in the synovial
  • Gout is a disease associated with an inborn error of uric acid metabolism that increases production or interferes with the excretion of uric
  • Cardiovascular Diseases
  • Cardiovascular diseases refer to a number of diseases associated with the blood vascular
  • Some major cardiovascular diseases are rheumatic heart disease, hypertensive heart disease and coronary heart
    • Rheumatic heart disease

Rheumatic heart disease is an autoimmune disease, most common in children after a severe throat infection by certain strain of Streptococcus bacteria.

An antigen on the surface of these bacteria is very similar to an antigen on the surface of myocardium. The antibodies against Streptococcus may react with myocardium and cause heart difficulties.

  • Hypertensive heart disease

Hypertensive heart disease are caused by hypertension, i.e., increased blood pressure. Serious hypertension is a common cause of chronic heart failure particularly in older people.

  • Coronary heart diseases

Coronary heart diseases are characterised by impaired heart function due to inadequate blood flow to the


Angina pectoris is the chest pain caused most often by myocardial anoxia.

Attacks of angina pectoris are often related to exertion, emotional disturbance and exposure to excess cold.

Myocardial infarction is commonly called coronary or heart attack.

Arteriosclerosis is the hardening of arteries due to deposition of cholesterol on arterial wall.

Coronary heart disease may be due to raised serum cholesterol, cigarette smoking, high blood pressure,

physical inactivity, obesity and diabetes.




Cyanosis refers to a bluish coloration of the skin and mucous membranes due to too much deoxygenated haemoglobin in the blood.

Cyanosis commonly can be noticed in finger nails, toe nails and lips. Irrational fear of disease is called pathophobia.

  Important Diseases.

(i) Important viral and Bacterial diseases

  • Important diseases caused by Viruses : The human diseases caused by viruses include influenza, chickenpox, smallpox measles, rabies, mumps, polio, trachoma, hepatitis and
    • Influenza : Influenza, commonly called flu, is a highly infectious disease, which has still not been It is caused by many kinds of viruses, such as myxovirus. The latter affect the mucous membrane of nose, throat and upper respiratory tract. The common symptoms are discharge from the nose, sneezing, fever, body aches, coughing and general weakness. The infection spreads by discharges from the nose and throat. The incubation period is just from 24-72 hours. Influenza generally lasts for 4 or 5 days. Rest quickens the recovery. If neglected, complications like pneumonia, bronchitis and ear infection may develop. There is no vaccine for influenza.

Influenza tends to occur in epidemic or pandemic form with varying virulence.

  • Chickenpox : It is a common, relatively mild, highly contagious disease of children, generally under 10 years of It is caused by a virus called chickenpox virus (varicella zoster). Fever, aches and general discomfort are the symptoms. Dewdrop-like sores appear in successive crops, first on the trunk. The sores open and a fluid seeps out a short time later. The disease spreads by direct contact with skin sores or with clothes and other articles soiled with discharges from sores. Incubation period is 2-5 weeks. The sores heal without leaving scars. Preventive measure is isolation of the patient till all crusts fall off. One attack of chickenpox ordinarily gives permanent immunity to the disease. There is no vaccine against chickenpox. Chickenpox is rarely fatal, but in adults attack could be severe.
  • Smallpox : Smallpox is an acute, highly communicable It is caused by a virus named variola virus. It starts as a sudden onset of high fever accompanied by headache, backache, and pains all over the body. Rash appears on the 3rd or 4th day of illness. The rash gradually changes into pustules (pimples) containing clear fluid. The pustules finally form scabs which fall off by the 3rd week. The scabs leave behind permanent pitted scars, the pockmarks, on the skin. The disease may lead to blindness.

Smallpox spreads by exudate from pustules on the skin of the infected persons. It also spreads by oral and nasal discharges during coughing and sneezing, and by contact with the clothes of the patient soiled with discharges. Its incubation period is about 12 days. It is very serious, disfiguring and highly fatal disease. It has now been largely controlled through vaccination. Smallpox vaccine was first prepared by Edward Jenner in 1798.

  • Measles : Measles is one of the most prevalent and serious diseases of children, generally 3-5years old. It is caused by a virus named rubeola virus. It is characterized by fever, inflammation of nasal mucous membrane, red watery eyes sensitive to light, flushed face, loss of appetite, followed by a typical rash, e., eruption of small red spots (rubeola). Infection is spread by discharges from nose and throat (droplet infection). The incubation period is about 10 days. One attack of measles gives life-long immunity. Vaccine which produces active immunity is available.

Patients of measles are likely to catch secondary infection of pneumonia.

  • Rabies (Hydrophobia) : Rabies is a 100% fatal disease. It is caused by a rabies virus. The virus enters the human body with saliva of an infected (rabid) animal, generally by the bite of a dog but also of cat. Virus induces biting behaviour in its victim. Fear of water is the main symptom, hence hydrophobia. Incubation period is commonly 1-3 months, but may vary from 10 days to one This long period of incubation makes it possible for a rabies vaccination after a bite to develop immunity and prevent the appearance of the disease. The virus of rabies destroys the brain and spinal cord cells. The patient feels severe headache, high fever, restlessness and inability to swallow even fluids due to choked throat. The main preventive measures are eradication of stray dogs and cats and




compulsory immunization of pet dogs and cats. The pet should be watched for 10 days after it has bitten someone to make sure that it does not have rabies. Symptoms of rabies in dogs are madness, changed voice and excessive salivation. Rabid dogs should be immediately killed. Treatment of rabies was discovered by Louis Pasteur. It involves a series of 14 injections given after the bite of a dog.

  • Mumps (Infectious Perotitis) : Mumps is an acute communicable disease, generally of It is caused by a paramyxo virus, which has preference for salivary glands but may attack other glands of the body also. It is characterized by painful enlargement of one or both the parotid glands. The latter lie below the pinnae. The patient has high fever and difficulty in opening mouth. The virus is spread by discharges from the throat of an infected person (droplet infection) and by direct contact. The incubation period varies from 12-26 days. In adults testes and ovaries may also become inflamed. Infection of testes may cause sterility. One attack of mumps gives life- long immunity.
  • Poliomyelitis or Polio (Infantile Paralysis) : Polio is most prevalent in hot, dry weather. Its common name is inappropriate as it is not necessarily a disease of infants nor does it always cause paralysis. It is caused by a virus known as polio virus. This virus causes inflammation of nervous system and stiffness of the neck. It also destroys motor nerve cells in the spinal Muscles fail to work and shrink due to lack of nerve impulses. This may cause paralysis of limbs in some cases. The virus enters the digestive tract with contaminated food and water and multiplies in the intestinal cells. It then passes into blood stream and lymphatic system, and finally reaches the spinal cord where it starts multiplication. Incubation period is 7-14 days. A patient who recovers from polio has a life time immunity. Now oral vaccine of polio is available.

The polio virus may attack the respiratory centres in the brain. This may stop nerve impulses to the diaphragm and breathing may stop. Then artificial breathing with ‘iron lung’ becomes necessary.

As polio cripples the children for life and is not curable, its prevention by oral vaccine is essential.

Oral vaccines are developed by Jonas Salk and Albert Sabin in 1940. Public pulse polio immunization programme is organized in India for eradicating polio in 1996.

  • Trachoma : Trachoma is a chronic inflammatory disease of the eyes all over the world. It is caused by a pathogen formerly considered a virus, new regarded an agent occupying a position intermediate between rickettsiae and viruses and named Chlamydia trachomatis. The latter affects eyelids, conjunctiva and cornea. It causes granules and may lead to blindness. The common symptoms are inflammation, discomfort and discharge from the eyes. Infection spreads by direct contact, by use of towels, pillows and handkerchiefs of the patients and by The incubation period is 5-12 days. Trachoma can be controlled with antibiotics in early stages. Severe infection needs operation-involving scrapping of granules. Trachoma accounts for 5 percent of the blind cases in India.
  • Dengue Fever (Backbone fever):

Dengue fever is a tropical viral disease spread by the tiger mosquito Aedes aegypti.

Dengue fever/Dengue Haemorrhagic Fever (DF/DHF), one of the dangerous diseases, can be detected by

tourniquet test.

The symptoms of this disease include high fever, severe frontal headache, pain behind eyes, muscles and joint pain, loss of appetite, Measles-like rashes over chest and upper limbs, nausea and vomiting.

Dengue can be prevented through elimination of mosquito breeding places.

  • Yellow fever

Yellow fever, caused by an arbovirus is a haemorrhagic disease transmitted by the infected Aedes aegypti.

Symptoms of yellow fever are headache, fever, vomiting, rapture of veins in kidneys, spleen, liver etc. In severe cases, the skin of sufferer becomes yellow from jaundice– hence the name yellow fever.

Max Theiler in 1951 got Nobel Prize for the development of vaccine for yellow fever.




  • Important Diseases Caused by Bacteria : The human diseases caused by bacteria include cholera, pneumonia, typhoid, tetanus, diphtheria, whooping cough, tuberculosis, plague, leprosy, syphilis, gonorrhoea, diarrhoea and

Bacterial diseases and their pathogens


Disease Causative Bacterium
(1) Cholera Vibrio comma (Vibrio cholerae)
(2) Pneumonia Diplococcus pneumoniae
(3) Typhoid Salmonella typhi
(4) Tetanus Clostridium tetani
(5) Diphtheria Corynebacterium diphtheriae
(6) Whooping cough Bordetella pertussis
(7) Tuberculosis Mycobacterium tuberculosis
(8) Plague Pasteurella pestis
(9) Leprosy Mycobacterium leprae
(10) Syphilis Treponema pallidium
(11) Gonorrhoea Neisseria gonorrhoeae
(12) Diarrhoeal Diseases Escherichia coli, Shigella dysenteriae, Campylobacter, Salmonella
(13) Anthrax Bacillus anthracis
  • Cholera : Cholera is an acute diarrhoeal disease. It is caused by a comma-shaped, motile bacterium called Vibrio comma or Vibrio cholerae. The organisms live in the intestine. Infection occurs with contaminated food and water. Incubation period varies from a few hours to 2-3 days. The symptoms of the disease are sudden onset of severe diarrhoea and vomiting. The stools are watery and give rice-water appearance. If the disease is not checked early, it leads to dehydration, loss of minerals, muscular cramps, suppression of urine and death. Rapid replacement of fluid and electrolytes is needed by oral rehydration therapy. Cholera epidemics are common in out country during fairs and floods and other natural calamities when water supply and sanitation go out of a Preventive measures include proper community sanitation, personal cleanliness, and taking boiled water and heated food. Cholera vaccine is useful during epidemic and visit to a fair. It, however, provides immunity for a short period, about 6 months. Visits to cholera affected places and families should be avoided. Vibrio cholerae first Isolate by Robert Koch in 1883.
  • Pneumonia : Pneumonia is a serious disease of the Lymph and mucus collect in the alveoli and bronchioles. With the result, the lungs do not get sufficient air to support life. The disease is caused by a bacterium Diplococcus pneumoniae. It usually follows lowered body resistance due to exposure or infection of some other disease such as influenza. Infection spreads by sputum of the patient. Incubation period is just 1-3 days. Pneumonia commonly occurs in old people.
  • Typhoid : Typhoid is characterized by constant fever. It is caused by a rod-like, motile bacterium named Salmonella typhi. The organisms live in the intestine and cause lesions in the intestinal wall. The disease spreads by contaminated food and Intestinal discharges of the patient contain the parasites. Incubation period varies from 1-3 weeks, average 2 weeks. Preventive measures include proper community sanitation, screening of water supply and food from contamination by flies, and personal cleanliness. Natural calamities like floods and hurricanes may cause epidemic of the disease. Typhoid vaccine provides immunity for about three years. Georges Fernand I. Widal (1896) devised the Widal Test for Diagnosis of Typhoid.
  • Tetanus (Lockjaw) : Tetanus is a major endemic recurring in a locality disease in our country. It is responsible for a high mortality of infants and their It is caused by anaerobic bacillus Clostridium tetani.




The bacillus enters the body through wounds and burns, and also by use of improperly sterilized surgical instruments. Incubation period varies from four days to three weeks. Tetanus results in painful muscular spasms and paralysis, which usually begins with jaw and neck muscles. This has led to the name “lockjaw”. The disease is often fatal.

Tetanus organisms live in the intestine of horses and other animals without doing any harm. The spores are, therefore, abundant in the soil manured with animal dung. They are also present in the road and street dust because the animals pass out dung as they move about. Spores may survive for 60 or more years in the contaminated soil. On entering the body by way of wounds, the spores release active bacteria. The latter multiply and secrete a powerful toxin tetanospasmin into the tissue, and blood carries it to the central nervous system. The toxin brings about tetanus.

It is advisable to have tetanus toxoid injection in case of an injury in a road accident or a cut contaminated with street dust or animal dung. This will prevent tetanus. All of us should have toxoid immunization as a safe preventive measure against this dangerous disease. Tetanus toxoid gives active immunity. Anti tetanus serum (A.T.S.) produces passive immunity. It is now a practice to immunize the infants against diphtheria, whooping cough (pertussis) and tetanus simultaneously by DPT or triple vaccine.

  • Diphtheria : Diphtheria is a serious disease of 2-5 years old It may attack adults also. It tends to occur in an epidemic form. It is caused by a rod-shaped bacterium named Corynebacterium diphtheriae. It commonly attacks the mucous membrane of nose, throat and tonsils. A semisolid material oozes from the affected region and forms a tough membrane over it. It may block the air passage. An acute case may need throat surgery. The bacteria may invade the heart, causing fatal heart blockade. The disease spreads by discharges from the affected regions (droplet infection). Incubation period is 2-5 days. Diphtheria antitoxin rids the victim of infection fully if given within 24 hours of the appearance of the symptoms. The symptoms include high fever, sore throat, difficulty in breathing due to choking. After 24 hour the antitoxin is not effective. Babies should be immunised with DPT vaccine within the first six weeks of birth.
  • Whooping Cough (Pertussis) : Whooping cough is primarily a disease of children. It is usually not serious in older children, but is often fatal in infants. It affects the respiratory tract. It is caused by a bacterium Bordetella pertussis. It spreads by discharges from the throat of infected person (droplet infection) and direct Incubation period is 10-16 days. Fever, severe coughing, vomiting and characteristic gasping “whoop” (loud, crowing inspiration) are common symptoms. Infants strangle from accumulation of mucus. Whooping cough vaccine (DPT) can immunize the infants.
  • Tuberculosis : Tuberculosis, commonly called T.B., is a very serious disease. About half a million people die of this disease each year in our country. It is especially common among poor people living in dingy, ill- ventilated, congested localities of big cities. It is caused by a rod-shaped bacterium named Mycobacterium tuberculosis. Tuberculosis (TB) or “consumption” is a bacterial disease caused by Mycobacterium tuberculosis. It commonly affects the lungs, where small tubercles are formed but may attack any part of the body, including the Infection spreads by sputum from the person suffering from the disease (droplet infection). Incubation period is quite variable. The bacteria damage tissues and release a toxin named tuberculin which produces the disease. Symptoms of pulmonary tuberculosis are fever, cough, blood-containing sputum, pain in the chest and loss of weight. Contrary to common belief, tuberculosis is curable. Treatment in early stages of the disease yields best results. It includes rest, good diet, drugs, surgery, health education and rehabilitation. BCG vaccine gives considerable protection against tuberculosis, but it should be used as a supplemental measure rather than to replace other measure of control. World T.B. Day is celebrated on 24 March.




  • Plague : Plague is essentially a disease of the rats, and is one of nature’s methods of periodically reducing the rat population. Man is affected incidentally. The disease is caused by a rod-shaped, nonmotile bacillus, Pasteurella pestis. It is carried from rat to rat by rat fleas, chiefly, Xenopsylla cheopis. The rat fleas leave the rats that die of plague, and bite human beings, thus infecting them with the disease. Death of the rats in a house may indicate the onset of Plague is normally not spread from man to man. The incubation period of plague is 2- 6 days. The disease is characterized by high fever, prostration (extreme weakness), and painful bubo (enlargement) of lymph nodes, generally in the groin or armpit. Plague has high mortality. A plague epidemic in Europe in 1348 reduced the population to one-third. Plague reached India in 1895 with ships from Hong Kong. Bubonic plague is caused by yersinia pestis (formerly pasteurella pestis) wayson stain test is used for susceptilbility of plague. Bubonic plague is basically a blood disease.

Preventive measures include killing the rats, having rat-proof ships and houses, killing the rat fleas when plague outbreak is suspected and immunization with plague vaccine.

  • Leprosy (Hansen’s Disease) : Leprosy is a chronic infectious disease, endemic in warmer climates. It is caused by a bacillus named Mycobacteriun leprae, which was discovered by It primarily affects the skin, mucous membrane and peripheral nerves, but may affect internal organs also. Its symptoms include hypopigmented skin patches, partial or total loss of sensation in the affected areas, lesions, ulcers, nodules, scales, deformity of fingers and toes, wasting of body parts, and thickened nerves. Infection occurs by prolonged and close contact with the leprosy patients. Babies isolated from leper parents early in life grow into normal healthy individuals. The bacilli leave the body in nasal discharge, from the throat during coughing, sneezing and even speaking, and through broken skin lesions. Incubation period is not exactly known. It is commonly between 2 to 5 years, but may vary from a few months to 30 or 40 years. Some 10.7 million people suffer from leprosy in Asia and Africa (WHO report). Leprosy has a special position among the communicable diseases because of the long duration of the disease, the frequency of disabilities and the social stigma it carries. It is a curable disease and the public should be educated about it and about the rehabilitation of the cured patients in society.
  • Sexually Transmitted Diseases (STD) : The sexually transmitted diseases, also called venereal diseases (VD), spread by sexual intercourse with infected The major venereal diseases are syphilis and gonorrhoea. These are international diseases. There are about 50 million cases of syphilis and 150 million cases of gonorrhoea in the world. However, the reported cases are merely a fraction of the actual prevalence of thesee diseases. The venereal diseases constitute a major medical problem in India.

Syphilis : Syphilis is caused by spirochaete bacterium, Treponema pallidium. It affects the mucous membranes in genital, rectal and oral regions, and causes lesions. Infection occurs by contact. Incubation period is about 3 weeks. The mothers may transmit the disease to their new-born babies. Syphilis is an easily curable disease. Syphilis is commonly known as “French disease” or “French pox” caused by a spirochete, Treponema pallidum.

Gonorrhoea : Gonorrhoea is caused by a diplococcus bacterium, Neisseria gonorrhoeae. The victim feels burning sensation during urination. Incubation period is 2 to 5 days. The disease affects the mucous membrane of the uriogenital tract, and spreads by sexual contact. The infection may spread to other parts of the body and cause arthritis and female sterility. The children born to afflicted mothers often suffer from eye infection (gonococcal ophthalmia). Gonorrhoea is also easily curable.

  • Diarrhoeal Diseases : These are a group of intestinal infections, including food poisoning. The prominent symptom of all such infections is diarrhoea. Infections spread through contaminated food, water, drinks, hands, clothes, bed sheets and utensils. The causative agents are mainly bacteria such as Escherichia coli, Shigella dysenteriae, Campylobacter and Salmonella. A protozoan Giardia intestinalis and some viruses also act as causative




agents. Toxins released by E. coli cause mild diarrhoea (loose and frequent evacuation of bowels) to severe dehydration. Shigellosis caused by Salmonella. The protozoans Giardia intestinalis and Balantidium coli and some viruses also act as causative agents. Toxins released by E. coli cause mild diarrhoea (loose and frequent evacuation of bowels) to severe dehydration. Shigellosis caused by Shigella is characterised by frequent passage of stools with blood and mucus and abdominal cramps. All diarrhoeal diseases caused dehydration, which can be countered with oral rehydration therapy, i.e., intake of adequate fluid and electrolytes.

Food infection should be distinguished from food poisoning. In food infection, food merely transfers bacteria into the body. In food poisoning, bacteria grow in food and release toxins. When such a food is taken, toxins are absorbed into the blood from the digestive tract. They affect the body quickly, causing gastrointestinal trouble and other effects. Clostridium botulinum is a Gram positive anaerobic bacillus responsible food poisoning known as botulism The bacilli release exotoxin to the environment, which is one of the most potent neurotoxic substance produced by microbs. Bubonic plaque is caused by yersinai pestis (formerly pasteurella pestis), a Gram-negative rod

  • Important Diseases Caused by Protozoans : Protozoans cause many diseases in The major ones in our country are amoebiasis, diarrhoea, ciliary dysentery and malaria. Some diseases are given below:
  • Amoebiasis (Amoebic Dysentery, Enteritis) : Amoebiasis is widespread in India due to poor sanitary conditions and polluted drinking water. The disease is caused by Entamoeba histolytica all over the world. The parasites live in the large intestine and lower part of the small intestine of Infection occurs by ingesting cysts with food and drinks.

The parasites secrete a proteolytic enzyme, cytolysin, that erodes the mucous membrane of the intestine. This may form bleeding ulcers that produce dysentery. In this disease, the patient passes out blood and mucus with the stools. He also experiences severe gripping pain in the abdomen, fever, nausea, exhaustion and nervousness. In chronic cases, the intestinal will is punctured. This may prove fatal. The parasites that invade the intestinal mucous membrane may be carried by the blood stream to the liver, lungs and brain. In these organs, the parasites, feed on cells and produce severe lesions and abscesses. The latter may cause death.

  • Diarrohea : Diarrohea is caused by a flagellate protozoan named Giardia intestinalis. Giardia was discovered by Leeuwenhoek in his own stools in 1681. It is the first human parasitic protozoan known. It is found all over the world. It inhabits the upper parts (duodenum and jejunum) of human small intestine all over the world. It lives firmly attached to the intestinal mucous membrane by adhesive disc, each perched on a separate cell. Nutrition is saprozoic, e., fluid food is absorbed through the body surface. Reproduction occurs by longitudinal binary fission. At intervals the parasites change into cysts which escape with the host’s faeces. Infection occurs by taking cysts with food and drinks. By covering the mucous membrane of the intestine, the parasites check or reduce the absorption of food, particularly fats. This causes diarrhoea or giardiasis (very loose and frequent stools).

Preventive Measures : Properly washing hands, fruits and vegetables before eating, and protecting the food articles from dust, flies, ants and cockroaches can check human infection.

  • Malaria : Malaria has been for thousands of years a very serious disease of the tropical and temperate It was almost eliminated a few years back with the efforts of World Health Organization (WHO) and our National Malaria Eradication Programme (NMEP), but unfortunately, it has appeared again.

Symptoms : The attack of malaria is preceded by yawning, tiredness, headache and muscular pain. During the fever, the patient feels chilly and shivers, and has acute headache, nausea and high temperature. After a few hours, the body perspires freely and the temperature becomes normal. The cycle is repeated if no medicine is taken. Blood smear made during fever shows the malarial parasites. No parasites are seen at other times. In chronic cases, there is general weakness and anaemia (paleness) due to large-scale destruction of red blood corpuscles. This is also accompanied by enlargement of spleen and liver.

Cause : Malaria is caused by the toxins produced in the human body by the malarial parasites, Plasmodium.




Transmission : The malarial parasites are carried from the infected to the healthy persons by the female Anopheles mosquito. The mosquito picks up the parasites with the blood, when it bites an infected person. When this infected mosquito bites a healthy person, parasites migrate into his blood with the saliva, which the mosquito injects before sucking up blood to prevent its clotting.

Types : There are four species of Plasmodium, which cause different kinds of human malaria –

  • Vivax : It causes benign tertian malaria, which attacks every third day, i.e., after 48 hours. The fever is mild and seldom fatal. This species is wide-spread in the tropical and temperate regions.
  • ovale : It also causes benign tertian malaria, which recurs every 48 hours. This species is found only in West Africa and South America.
  • malariae : It causes quartan malaria, which recurs every fourth day, i.e., after 72 hours. This species is found in both tropical and temperate regions, but it is not very common.
  • falciparum : It alone is capable of causing three types of malaria, viz., quotidian malaria, which attacks almost daily, malignant tertian malaria, which occurs every 48 hours, but is very severe and often fatal; and irregular malaria. This species is found only in the tropical region.

Incubation Period : The incubation period for malaria caused by Plasmodium vivax is about 10 days.

History : The name malaria was given by Mucculoch in 1872 on the belief that it was caused by the foul air of the marshy localities (Italian mala = bad, aria = air). In 1880 Laveran, a French army medical officer, discovered the malarial parasites in the blood of a malarial patient. Sir Ronald Ross of the Indian Medical Service established the “mosquito-malaria relationship” on August, 29 1897, ever since called the “Mosquito Day”.

Life-history : Plasmodium completes its life cycle in two phases and two hosts : asexual phase in the human host and sexual phase in the female Anopheles mosquito host.

  • Ciliary Dysentery : Ciliary dysentery is caused by a ciliate protozoan named Balantidium coli. The latter inhabits the human large intestine (colon) all over the It feeds on tissue fragments, red blood corpuscles, bacteria and faecal matter. It reproduces asexually by transverse binary fission and sexually by conjugation. The latter is followed by cyst formation. Cysts pass out in the host’s faeces. Infection occurs by ingesting cysts with food and drinks. Balantidium coli causes ulcers in the colon and invades mucous membrane by secreting cytolysin. This generally results in diarrhoea, but may lead to severe or fatal dysentery.
  • Important Diseases Caused by Helminthes : Helminthes (flatworms and roundworms) cause many diseases in The more common are taeniasis, ascariasis and filariasis (elephantiasis).
  • Taeniasis : Taeniasis is caused by the pork tapeworm Taenia solium. This tapeworm lives in the human intestine, firmly anchored by hooks and suckers. It lacks mouth and absorbs host’s digested food through its skin (saprozoic nutrition). It is hermaphrodite and undergoes self-fertilization. There is normally a single worm in one This worm has enormous power of reproduction.
    • Life-history : Taenia solium has about 4 metres long, white, flat, ribbon like body comprising a small knob-like scolex, a short neck and a very long strobila of about 850 proglottides. Capsules of the worm pass out in host’s faeces and are ingested by They release embryos which reach the pig’s striated muscles, encyst and develop into infective larvae. There may be about 3,000 larvae in 500 grams of pork. The infected pork has brownish spots and is called “measly pork”. Man gets infection by taking raw or undercooked measly pork. Pink or red appearance of the pork, when cut into slices, is an indication of its being undercooked. In the human intestine, the cyst wall breaks down, releasing the larva. The latter grows into an adult worm in 3 to 4 months.
    • Gravid proglottids : These segments are pregnant uterus, which possess fertilized ova, and all structures disintegrate in it the dropping of gravid proglottids is called




  • Effect on the Host (Pathogenicity) : The tapeworm infection produces little effect on a person with a sound Weak person may develop a disease named taeniasis. This disease is characterized by abdominal pain, indigestion, vomiting, constipation, loss of appetite (anorexia) and weight, insomnia, lowered resistance to other diseases and nervous disorder.
  • Cysticercosis : Tapeworm infection can also occur by taking improperly washed vegetables and water contaminated with the tapeworm Thus, the vegetarians can also get tapeworm infection. The capsules release embryos which may reach eyes or brain, develop into larvae, called cysticerci, that encyst. The cysts so formed may cause blindness or epilepsy-like symptoms and prove fatal.
  • Ascariasis : Ascariasis is caused by the roundworm Ascaris lumbricoides. This roundworm lives in the human small It lies free, having no organs for attachment. It takes host’s digested food by sucking through the mouth (holozoic nutrition).
    • Life-history : Man gets infection by taking Ascaris eggs with food and Children become infected by ingesting soil. Eggs hatch in the host’s intestine in a few hours, each liberating a tiny (0.2 to 0.3 mm. long) worm called juvenile. The latter grows into an adult worm in 2 to 2.5 months. The adult worm has cylindrical body tapering at each end, 20 to 40 cm. long in female and 15-30 cm. long in male. Male’s hind end is curved ventrally. Mature male and female worms copulate in the host’s intestine, where the female later lays eggs. The eggs pass out in the faeces, and can remain alive in the soil for several years. The eggs are carried to food and drinking water by air, flies and cockroaches.
  • Filariasis (Elephantiasis) : Filariasis is caused by the filarial worm, Wuchereria bancrofti.
    • Life-history : The adult male and female worms are 40 mm. and 80 mm. long They live in the lymphatics and connective tissues. The worm is viviparous. The female delivers young worms called microfilariae. The latter shift to deep blood vessels. At night they migrate to the superficial blood vessels of the skin and are sucked by Culex mosquito, the intermediate host. The mosquito injects them into the blood of a healthy human being. Form the blood, they migrate to the lymph vessels and lymph glands. Here they grow into adults in about a year.
    • Effect on the Host (Pathogenicity) : In acute cases the filarial infection causes fever. In chronic cases the worms block the lymph vessels. This causes enormous swelling of the affected part, which may be foot, leg, or This is followed by thickening of skin and subcutaneous tissue. Enlargement of legs gives the disease its name, elephantiasis.
  • Ancylostomiasis (Hookworm Disease) : Ancylostomiasis is caused by the hookworm, Ancylostoma duodenale. It lives in the small intestine firmly attached to its It feeds on blood and bits of mucous membrane. A secretion from its pharyngeal gland prevents clotting of blood while the worm is feeding and causes considerable loss of blood after the worm has left the wound. Eggs laid by the female worm in the host’s intestine escape with the faeces and hatch in the moist soil. The larvae feed on organic debris and get into the human body by boring through the skin of the feet, causing “ground itch.” They enter the veins, and passing through the heart, lungs, trachea, pharynx and oesophagus, reach the intestine. Here, they mature. Adult worms live for about 5 years. Male worm is 8-11 mm. long, and female 10-13 mm.

(v) Sexually Transmited disesases

  • Acquired Immune Deficiency Syndrome (AIDS) : AIDS was first noticed in USA in 1981. It is a disorder of cell-mediated immune system of the There is a reduction in the number of helper T-cells which stimulate antibody production by B-cells. This results in the loss of natural defense against viral infection. It is caused by a virus named HIV (Human Immunodeficiency Virus). The virus was discovered in 1984 by American and French scientists independently. It is heartening to know that only 10% of the people who get AIDS virus infection actually develop full-blown AIDS. World AIDS day is celebrated on 1st December.




AIDS, however, is primarily a sexually transmitted disease. Semen can contain the virus, but more likely an infected lymphocyte does, About 64% of the

total case in the United States are homosexual men who practice anal intercourse. Unlike the vagina, the epithelial lining of the rectum is a thin, single-celled layer that is easily torn during intercourse. Nevertheless, heterosexual transmission does occur and may become more prevalent as more females become infected. One unhappy side effect to female infection is the fact that viruses and infected lymphocytes can pass to a foetus via the placenta or to an

infact via the mother’s milk. Presently, infected infants account for about 2% of all AIDS cases.

Symptoms of AIDS : An HIV infection can be divided into 3 stages.

  • Asymptomatic Carrier : Only 1%-2% of those newly infected have mononucleosis-like symptoms that may include fever, chills, aches, swollen lymph glands, and an itchy rash. These symptoms disappear, and there are no other symptoms for 9 months or Although the individual exhibits no symptoms during this stage. He or she is highly infectious. The standard HIV blood test for the presence of antibody becomes positive during this stage.
  • AIDS Related Complex (ARC) : The most common symptom of ARC is swollen lymph glands in the neck, armpits, or groin that persist for 3 months or more. There is severe fatigue unrelated to exercise or drug use; unexplained persistent or recurrent fevers, often with night sweats; persistent cough not associated with smoking, a cold, or the flu; and persistent Also possible are signs of nervous system impairment, including loss of memory, inability to think clearly, loss of judgment, and/or depression.

When the individual develops non-life threatening and recurrent infections such as thrush or herpes simplex, it is a signal that full-blown AIDS will occur shortly.

  • Full-Blown AIDS : In this final stage, there is severe weight loss and weakness due to persistent diarrhoea and usually one of several opportunistic infections is These infections are called opportunistic because the body can usually prevent them – only an impaired immune system gives them the opportunity to get started. These infections include the following –

Pneumocystis carinii pneumonia : There is not a single documented case of this type of pneumonia in persons with normal immunity.

Toxoplasmic encephalitis : In AIDS patients, this infection leads to loss of brain cells, seizures, and weakness.

Myobacterium avium : This is an infection of the bone marrow that leads to a decrease in red blood cells, white blood cells, and platelets.

Kaposi’s Sarcoma : A cancer of the blood vessels that causes reddish purple, coin-size spots and lesions on the skin.

  • Treatment of AIDS : The drug zidovudine (also called azidothymidine, or AZT) and dideoxyinosine (DDI) prevent HIV reproduction in cells. Proteases are enzymes HIV needs to bud from the host cell; researchers are hopeful that a protease inhibitor drug will soon be

A number of different types of vaccines are in, or are expected to be in, human trials. Several of these are sub unit vaccines that utilize genetically engineered proteins that resemble those found in HIV. For example, HIV-1, the




cause of most AIDS cases has an outer envelop molecule called GP 120. When GP 120 combines with a CD4 molecule that projects from a helper T lymphocyte, the virus enters the cell. There are sub unit vaccines that make use of GP 120. An entirely different approach is being taken by Jonas Salk, who developed the polio vaccine. His vaccine utilizes whole HIV-1 killed by treatment with chemicals and radiation. So far, this vaccine has been found to be effective against experimental HIV-1 infection in chimpanzees, and clinical trials will occur soon.

AIDS Prevention : Shaking hands, hugging, social kissing, coughing or sneezing and swimming in the same pool do not transmit the AIDS virus. You cannot get AIDS from inanimate objects such as toilets, doorknobs, telephones, office machines, or household furniture.

HIV has been isolated from semen cervical secretions, lymphocytes, plasma, cerebrospinal fluid, tears, saliva, urine and breast milk. The secretions known to be especially infectious are semen, cervical secretions, blood and blood products. Infection is spread :

  • By sexual intercourse, vaginal and anal
  • By infected blood, blood products, donated semen and organs
  • By contaminated needles used :
  • During the treatment of patients
  • When drug abusers share needles
  • From an infected mother to her child :

(1 ) Across the placenta before birth

  • While the baby is passing through the birth canal
  • Possibly by breast milk

The presence of antibodies to HIV indicates that the individual has been exposed to the virus but not that a naturally acquired immunity has developed. All those who have antibodies in their blood do not develop AIDS although they may spread the infection to others. A few weeks after infection there may be an acute influenza-like illness with no special features, followed by a period of two or more years without symptoms. When AIDS Develops the main complications are widespread recurrent opportunistic infections and tumours. Outstanding features include:

  • Pneumonia, commonly caused by Pneumocystis carinii, but many other microbes may be
  • Persistent nausea, diarrhoea and loss of weight due to recurrent infections of the alimentary tract by a wide variety of
  • Recurrent meningitis, encephalitis and brain abscesses, caused by opportunistic microbes and possible by HIV, followed by deterioration in neurological functions, characterised by forgetfulness, loss of concentration, confusion, apathy, dementia, limb weakness, ataxia,
  • Widespread skin eruptions, g., eczema, psoriasis, cellulitis, impetigo, warts, shingles, ‘Cold sores’.
  • Generalised lymphadenopathy, e., noninfective enlargement of lymph nodes.
  • Malignant tumours,
  1. lymphomas, i.e. tumours of lymph glands.
  • Kaposi’s sarcoma, consisting of tumours under the skin and in internal organs.

The following behaviour will help prevent the spread of AIDS :

  • Do not use alcohol or drugs in a way that prevents you from being in control of your Especially, do not inject drugs into veins, but if you are an intravenous drug user and cannot stop your behaviour, always use a sterile needle for injection or one cleansed by bleach.




  • Refrain from multiple sex partners, especially with homosexual or bisexual men or intravenous drug users of either Either abstain from sexual intercourse or develop a long-term monogamous (always the same partner) sexual relationship with a partner who is free of HIV and is not an intravenous drug user.
  • If you uncertain about your partner, always use a latex condom. Follow the directions, and also use a spermicide containing nonoxynol-9, which kills viruses and virus-infected The risk of contracting AIDS is greater in persons who already have a sexually transmitted disease.

Diagnosis : Once the host is infected by HIV. HIV detected by the ELISA Test. (Enzyme–linked immunosarbent assay a positive Elisa should be can firmed using another test called the western blot test.

  • Hepatitis : It is a liver inflammation caused by virus, use of many drugs, chemicals and alcohol. Hepatitis may be of following types :

Hepatitis A : It is caused by Hepatitis A virus. It is transmitted through infected food, water, clothes and faecaes. It may occur in epidemic form especially in areas where hygiene is poor. This virus does not damage liver cells.

Hepatitis B : It is caused by Hepatitis B virus. It is transmitted by infected food and blood products; such as plasma or by medical instruments contaminated with infected blood. It results in the swelling of liver cells.

Hepatitis is also caused by poisonous chemicals, alcohol, as a side effect of certain drugs and from severe amoebiasis.

Complete bed rest and protein free diet is the only recommended treatment. An intramuscular injection of

Gamma globulin can protect against infectious hepatitis for about 6 months.

Infectious hepatitis, also called epidemic jaundice, is inflammation of liver due to infection by two viruses : infectious hepatitis virus and serum hepatitis virus. It is an acute communicable disease and may be fatal. An attack of viral hepatitis usually provides protection against a second attack. Incubation period is usually 20 to 35 days. Infection spreads from person to person by faecal-oral route. Contamination of water and food may cause epidemic. Liver cells are damaged, releasing bilirubin that causes jaundice.

The control measures are (i) sanitary disposal of excreta; (ii) prevention of contamination of water, food and milk; (iii) control of flies; (iv) screening of kitchens and latrines; and (v) personal cleanliness and also that of food handlers. During epidemic, boiled or chlorinated water should be taken.


  • Cancer : Cancer is an abnormal and uncontrolled division of cells, known as cancer cells, that invade and destroy the surrounding tissues. Generally Cancer is defined as uncontrolled proliferation of cells without any differentiation. Cancer cells are different from normal cells in some aspects. They do not remain confined to one part of the body. They penetrate and infiltrate into the adjoining tissues and dislocate their Some of the cancer cells get detached from the main site of origin and travel by blood and lymph to sites distant from the original tumour and form fresh colonies, called metastasis or secondary growth.

Neoplasms or Tumours : A neoplasm (new growth) is a mass of tissue that grows in excess of normal in an uncordinated manner and continues to grow after the initial stimulus has ceased. Tumours are classified as benign or malignant.

Oncology : (G. onkos – mass, tumour; logos – study of) is the field of biomedicine devoted to the study and treatment of tumours.

  • Types of Tumours : There are two types of tumours : benign and malignant.
    • Benign Tumour (=Nonmalignant Tumour) : It remains confined to the site of its origin and does not spread to other parts of the It causes limited damage to the body. It is non-cancerous.




  • Malignant Tumour (= Cancerous Tumour) : It first grows No symptoms are noticed. This stage is called the latent stage. The tumor later grows quickly. The cancer cells go beyond adjacent tissue and enter the blood and lymph. Once this happens, they migrate to many other sites in the body where the cancer cells continue to divide. It is metastasis. Only malignant tumours are properly designated as cancer.

Differences between Benign Tumour and Malignant Tumour


Benign Tumour Malignant Tumour
(1) It remains confined to the affected organ. (1) It also spreads to other organs of the body.
(2) Rate of growth is usually slow. (2) Rate of growth is usually rapid.
(3) There is no latent stage. (3) There is latent stage.
(4) It causes limited damage to the body. (4) The cancer cells migrate to other sites of the body.
(5) There is no metastasis. (5) There is metastasis.
(6) It is non-cancerous. (6) It is cancerous.
  • Types of Cancer (Types of Malignant Tumours) : Malignant tumours are generally classified into three main types on the basis of cell type from which they
    • Carcinomas : This type is mainly derived from epithelial They include cervical (cervix is part of uterus) cancer, breast cancer, skin cancer, brain cancer, lung cancers, stomach cancer, etc.
    • Sarcomas : These cancers are located in connective and muscular tissues derived from mesoderm. Thus, they include the cancers of bones, cartilages, tendons, adipose tissue, lymphoid tissue and muscles. Cancer of bones is called Cancers of adipose tissue are known as lipomas and cancers of lymphatic tissue are termed as lymphomas. Hodgkin’s disease is an example of human lymphoma. In Hodgkin’s disease there is chronic enlargement of the lymph nodes, and enlargement of spleen and often the liver. In this disease there is excessive production of lymphocytes by lymph nodes and spleen.
    • Leukaemias (Blood cancers) : They are characterized by abnormal increase of white blood corpuscles count due to their increased formation in the bone


  • Causes of Cancer : The causes of cancer are not fully understood. However, many factors are known to favour cancer development. These factors are called carcinogenic agents or Carcinogens. The causes of cancer are briefly described under the following
    • Physical irritants : (i) Use of Kangri (an earthen pot containing burning coal) by Kashmiris causes abdominal skin cancer as these people keep Kangri close to their abdomen during (ii) Betal and tobacco chewing causes oral cancer. (iii) Heavy smoking causes lung cancer and may also cause cancer of oral cavity, pharynx (throat) and larynx. (iv) Jagged teeth may cause tongue cancer. (v) Excessive exposure to sun light can cause skin cancer.
    • Chemical Agents : Several chemicals are known to cause cancer. These are caffeine, nicotine, products of combustion of coal and oil and pesticides; constant use of artificial sweetener can cause cancer. An animal protein-rich diet is known to cause cancer of large Breast cancer has hormonal relationship. Thus, some sex hormones and steroids if secreted or given in large amounts may cause cancer. Chimney sweepers can develop cancer of scrotum. Dye workers have a high rate of bladder cancer.




Carcinogens and Organs Affected


Carcinogens Organs Affected
(1) Soot Skin, lungs
(2) Coaltar (3, 4-benzopirene) Skin, lungs
(3) Cigarette smoke (N-nitrosodimenthylene) Lungs
(4) Cadmium Oxide Prostate gland
(5) Aflatoxin (a mould metabolise) Liver
(6) 2-naphthylamine and 4-aminobiphenyl Urinary bladder
(7) Mustard gas Lungs
(8) Nickel and Chromium compounds Lungs
(9) Asbestos Lungs, pleural membrane
(10) Diethylstibestorol (DES) Vagina
(11) Vinylchloride (VC) Liver


  • Radiations : The X-rays, cosmic rays, ultra-violet rays, etc. can cause cancer. Japanese people exposed to radiations from World War II nuclear bombing show five times the incidence of leukemia seen in the rest of the
  • Biological Agents : Certain viruses can cause The viruses that cause cancers are called


Oncogens : It has now been confirmed that all cells carry some cancer-causing genes called oncogenes. Certain factors stimulate oncogenes to replicate rapidly, causing malignant tumour. Experts in the study of cancer are called oncologists.

Other term associated with cancer cell:

Melanama        :         Cancer of pigmented cells of skin. Adenoma         :         Cancer of gland.

Myoma            :         Cancer of muscular tissue. Lymphoma      :         Cancer of Lymphatic tissue. Glioma            :         Cancer of Glial cells of CNS.

Different Sites of Cancer : Some of the important sites of cancer are skin, mouth, oesophagus, stomach, colon, rectum, liver, gall bladder, pancreas, blood, lymph, adipose tissue, lung, uterine cervix, breast, brain, penis, prostate, muscles, thyroid, kidney and bones.

Possible Symptoms of Cancer : (i) A persistent cough or hoarseness in a smoker. (ii) A persistent change in digestive and bowel habits. (iii) A change in a wart or mole. (iv) A lump or hard area in the breast. (v) Unexpected diminished or lost appetite. (vi) Unexplained low-grade fever. (vii) Unexplained loss of weight. (viii) Any ulcer that does not get well. (ix) Bleeding in vagina at times other than the menstruation. (x) Non-injury bleeding from the surface of the skin, mouth or any other opening of the body.

  • Treatment : Three general methods of treatment for cancer are currently
    • Surgery : It involves the removal of the entire cancerous
    • Radiation : It involves the exposure of the cancerous parts of the body to X-rays, which destroy rapidly growing cells without harming the surrounding




  • Chemotherapy : It involves the administration of certain anticancer drugs. These drugs check cell division by inhabiting DNA These drugs may be more toxic to cancerous cells than to normal cells.

Most cancers are treated by combination of surgery, drugs and radiation therapy.

  • Theories related to Cancer : The theories that seem most worth investigation are mutation and selective gene
    • Mutation Theory : Evidences for the Mutation Theory are as follow : (i) Agents that are known to cause mutations (radiations and chemicals) also appear to cause cancer. (ii) The incidence of cancer increases with As the number of body cell mutations also increases with age, it is possible that cumulative effects of mutations contribute to the initiation of malignancy. However, the mutation theory fails to explain occasional cases of spontaneous remission. Remission is the condition in which symptoms and evidence of the disease disappear.
    • Selective Gene Activation : A second theory, that of selective gene activation, does account for If certain genes that are not normally expressed suddenly become active, their expression could lead to uncontrolled cell division. A remission might occur when for some reason these genes cease to be expressed. Research into the mechanism that control gene activation may provide insight into both the process of normal cell division and the aberrations in the process that lead to cancer.
    • Characteristics of Cancer Cells

(1) Nucleus is abnormally enlarged and irregular.             (2) Chromatin material is also irregular.

(3) ER are more in cancerous cells.                             (4) Ribosomes fuse together to form polyribosomes.

(5) Golgi bodies are less developed.                            (6) Mitochondria are swollen with few cristae.

(7) Plasma membrane often becomes irregular.              (8) Pathological cytoplasmic inclusions are also present.

  • Danger Signals for Cancer : These are as follows:

(1) Any wound that does not heal.                              (2) A thickening or lump in the breast or elsewhere.

(3) Any change in the mole or wart.                            (4) Unusual bleeding or discharge.

(5) Persistent indigestion or difficulty in swallowing.           (6) Persistent cough or hoarseness.

(7) Any change in normal bowl habits.

  • Types of Cancer : Cancers are generally named against the tissues These are –

(1) Carcinoma : Cancer of epithelial cells.                   (2) Sarcoma : Cancer of connective tissue.

(3) Leukaemia : Cancer of blood cells.                       (4) Lymphoma : Cancer of lymphocytes.

(5) Lipoma : Cancer of adipose tissue.

Pathogenic Protozoa


S. No. and name of parasite Host and site of parasite in its body Diseases caused Method of transmission
Class Rhizopoda      
(1) Entamoeba histolytica In the colon of man, sometimes in dogs and cats also. It may reach liver, spleen, lungs and brain etc. Amoebic   dysentry.  It               also causes ulcers in the Intestine. By contaminated food and water.
(2) Entamoeba coli In the colon of man. Gastro-intestinal disturbances. By contaminated food and water.
(3) Entamoeba gingivalis In the buccal cavity of man. Bleeding gums. By mouth contact.




Class Mastigophora      
(4) Trypanosoma gambiense In the blood of Africans. African sleeping sickness. By the bite of the fly, Glossina palpalis.
(5) Trypanosoma rhodesiense In the blood of Africans. Rhodesian sleeping sickness. By the bite of the fly, Glossina morsitans.
(6) Trypanosoma cruzi In early stages, it is found in the muscles, heart, brain, spinal cord and gonads of children but in later stages in the blood Chaga’s disease. By a bug.
(10) Leishmania donovani In the liver, lymph glands and leucocytes of man, dog and cat. Kala-azar fever. By    sand   fly, Phlebotomus


(11) Leishmania infantum In the spleen of children Enlargement of spleen. By    sand   fly, Phlebotomus


(12) Leishmania tropica In    endothelium   of       blood capillaries of skin of man Oriental sore. By    sand   fly, Phlebotomus


(13) Leishmania brasiliensis In the infected man, dog and cat. Skin   disease  (Espundia in man). By sand fly, Phlebotomus and contact.
(14) Trichomonas buccalis In the infected gums of man. Associated with pyorrhoea. By infected food.
(15) Trichomonas hominis In colon of man and other vertebrates. Associated with dysentry. By contaminated eatables and water.
(16) Trichomonas vaginalis In urinogenital tract of women. Vaginitis. During sexual intercourse.
(17) Giardia intestinalis In small intestine of man Diarrhoea. By contaminated food.
Class Sporozoa      
(18) Plasmodium vivax In erythrocytes and liver of


Different types of malaria fever. By the bite of female

Anopheles mosquito.

(19) Plasmodium falciparum
(20) Plasmodium malariae
(21) Plasmodium ovale
(22) Babesia bigemina In erythrocytes of cattle. Taxas fever and diarrhoea. By the bite of fleas.
(23) Isospora hominis In small intestine of man. Diarrhoea and other gastric troubles. By contaminated food.
(24) Eimeria stiedae In cells of mucous membrane of hepatic ducts and liver of rabbit. Diarrhoea and liver disorders. By their oocysts.
Class Ciliata      
(25) Balantidium coli In colon of human beings. Ulcers in colon and diarrhoea. By spores.


Viral Diseases in Humans


Disease Pathogen Habitat Main Symptoms Mode of Infection I.P.
Influenza Myxo viruses Mucous membrane of respiratory tract Nasal          discharge, sneezing, coughing By     droplets nose & throat from 24 to 72 hours
Smallpox Variola virus   Skin rash changing pustules, then to scabs to By contact, droplets and fomite 12 days
Chicken pox Varicella zoster   Skin sores that open & emit fluid By      contact fomite and 2 to 5 weeks




Measles Rubeola virus   Red watery eyes, skin rash By     droplets        from nose & throat 10 days
Rabies (Hydrophobia) Rabies virus Brain & spinal cord cells Biting behaviour, fear of water, inability to swallow Bite by rabid dog 1 to 3 months
Mumps (Infectious parotitis) Paramyxo virus Salivary glands Painful enlargement of parotid glands, difficulty in opening mouth By contact and droplets from throat 12 to 26 days
Poliomyelitis (polio) Polio virus Nerve cells Inflammation of nervous system, muscle shrinkage, limb paralysis By contaminated food & water 7 to 14 days
Trachoma Chlamydia


Eyelids, conjunctiva & cornea of eye Granules on inner surface of eyelids, watery eyes By contact and fomite 5 to 12 days
Acquired   immune deficiency syndrome (AIDS) Human immunodeficiency virus   Infections, cancer, brain damage, WBC destruction By contact with blood 28 months average,
Hepatitis viral (Epidemic jaundice) Infectious & serum hepatitis viruses Liver Jaundice due to damaged liver cells By contaminated food and water 20-35 days


Bacterial Disease in Human


Disease Pathogen Habitat Main Symptoms Mode of Infection I.P.
Cholera Vibrio         comma (V.cholerae) Intestine Severe diarrhoea and vomiting By        contaminated food and water 2 to 3 days
Pneumonia Diplococcus pneumoniae Lungs Difficulty in breathing By patient’s sputum 1 to 3 days
Typhoid Salmonella typhi Intestine Constant fever By        contaminated food and water 1 to 3 weeks
Tetanus (Lockjaw) Clostridium tetani Tissues Painful muscular spasms and paralysis Through               wounds and burns 4 days to 3 weeks
Diphtheria Corynebacterium diphthriae Mucous membrane of nose, throat & tonsils Sore throat, difficulty in breathing By oral & nasal discharges 2 to 5 days
Whooping cough (pertusis) Bordetella pertussis Respiratory tract Severe coughing characteristic gasping ‘whoop’ By throat discharges and contact 10 to 16 days
Tuberculosis Mycobacterium tuberculosis Lungs Cough, bloody sputum, chest pain By patient’s sputum Variable
Plague Pasteurella pestis Blood and lymph Painful pubo of lymph nodes By rat-flea bite 2 to 6 days
Leprosy Mycobacterium leprae Skin mucous membranes, peripheral nerves Hypopigmented skin patches, ulcers, deformity of digits Long and close contact with patients 2 to 5 years
Syphilis Treponema pallidium Oral, genital, rectal mucosa Lesions By contact 3 weeks
Gonorrhoea Neisseria gonorrhoeae Urinogenital mucosa Burning   sensation  in micturition By sexual contact 2 to 5 days
Diarrhoeal diseases Shigella dysenteriae, Salmonella, Intestine Diarrhoea By contaminated food, water, hands, fomite  




  Escherichia                        coli, Campylobacter        


Important Helminth Diseases in Humans


Disease Pathogen Habitat Mode of Infection
Taeniasis & Cysticercosis Taenia solium – the pork tapeworm Intestine By taking raw or undercooked measly pork
Ascariasis Ascaris lumbricoides Small intestine By taking eggs with food and water



bancrofti – the flarial worm

Lymphatics and connective tissue By bites of Culex mosquitoes
Ancylostomiasis (Hookworm disease) Ancylostoma duodenale – the hookworm Small intestine By boring through the skin, usually of feet.


Sexually Transmitted Diseases (STD) in Human


Disease Causative organism Nature of Disease Symptoms – Treatment
(1) AIDS (Acquired Immuno deficiency Syndrome) Retrovirus – HIV Viral Enlarged lymph nodes, long fever, weight loss – Nil
(2) Genital Herpes Herpes simplex virus Viral Painful ulcer on genitals – Nil
(3) Genital warts Human papilloma virus (HPVs) Viral Tumor of the vulva, vagina, anus and penis – Nil
(4) Gonorrhoea Neisseria gonoerrheae Bacterial Infection of all genital organs or PID – Penicillin
(5) Chlamydiasis Chlamydia trachomatis Bacterial White patches on vagina or PID – Nystatin
(6) Syphilis Treponema pallidum Bacterial Cancer and skin eruption – Benzene and Penicillin
(7) Trichomoniasis Trichomonas vaginalis Protozoan Greenish-yellow vaginal discharge– Metronidazole.
(8) Chancroid Haemophilus ducreyi   Foul discharge and ulcer Drug : Sulphonamide
(9) Lymphogranuloma venerum Lymphogranuloma psittacosis bacteria   Inguinal lymphadenopathy Drug : Tetracycline

Insect carrying diseases


Common name Zoological name Causative organism Disease
Mosquitoes Anopheles sps Culicine sps Stegomyia sps

Aedes aegypti

Plasmodium Wuchereria bancrofti Flavovirus fibricus

Dengue virus

Rat flea Xenopsilla cheopsis Pasteurella pestis Bubonic plague




  Xenopsilla sps R. typhi Endemic typhus
Flies Musca sps 1.       Shigella sps

2.       Salmonella typhi

3.       Salmonella paratyphi

4.       Hepatitis type – A virus

Bacillary dysentery Typhoid fever Paratyphoid fever

Infectious hepatitis

Sand fly Phlebotomus papatasi



Leishmania donovani

Sand fly fever

Kala azar

Body louse Pediculus Rickettsia prowazeki

R. Quintana

Trench fever
Mite Trombicula akamushi R. Tsutsugamushi Scrub typhus (Tsutsugamushi fever)
Itch mite Sarcoptes scabieri Scabies
Tick fever, Amblyomma sps R. rickettsiae Rocky mountain spotted theileriosis
House fly Musca domestica Vibrio cholerae



Infantile diarrhoea

Bed bug Cimex Relapsing fever
Tse-tse fly Glossina palpalis Trypanosoma gambiense Sleeping sickness


 Defense mechanism.

Immune response : Nature has provided certain ways in the body to defend ourselves from the invention of pathogens and therefore, from the disease. The ability of a host’s body to prevent or overcome the effects caused due to the invention by pathogenic organisms and its toxins is known as resistance and immunity. Resistance is considered as an inherent factor and those acquired during life to overcome the disease, while the immunity is accepted to be due to the acquired factors that help in resistance. The host body has two lines of defence that must be overcome by a pathogen before establishing an infection.

  • External defence mechanism : This defence mechanism involves mechanical and chemical factors g. skin, mucous membrane, mucous secretion, peristalsis, coughing, sneezing, shedding tears, etc. Chemicals are lysozymes present in the body.
  • Internal defence mechanism : This mechanism of defence has two lines of defence against pathogen :
  • A non specific mechanism comprising physical and chemical barriers and inflammatory reactions, and
  • A specific defence mechanism, the immune
  • Non-specific Defence Mechanism : It is further of two types : external defence or first line of defence and internal defence or second line of
    • External Defence : It includes physical and chemical

Physical Barriers

Skin : The skin is physical barrier of body. Its outer tough layer, the stratum corneum prevents the entry of bacteria and viruses.

Mucous Membrane : Mucus secreted by mucous membrane traps the microorganisms and immobilises them. Microorganisms and dust particles can enter the respiratory tract with air during breathing which are trapped in the mucus. The cilia sweep the mucus loaded with microorganisms and dust particles into the pharynx (throat). From the pharynx it is thrown out or swallowed for elimination with the faeces.





Chemical barriers

Oil secreted by the oil glands and sweat secreted by sweat glands make the surface of the skin acidic (ph 3-5). This does not allow the microorganisms to establish on the skin. Some friendly bacteria also occur on the skin which releases acids and other metabolic wastes that check the growth of pathogens. The sweat also contains an enzyme named lysozyme that destroys the cell walls of many bacteria.

Lysozyme is also present in tears and checks eye infections. Lysozyme is also present in the saliva which kills bacteria of food. Highly acidic gastric juice also kills harmful bacteria in the stomach Bile checks the growth of foreign bacteria in the intestine.

The mesh of fine hair in our nostrils filters out particles which may carry pathogens. Nasal secretions also destroy the harmful foreign germs with their lysozyme.

Certain bacteria normally live in vagina. These bacteria produce lactic acid. Lactic acid kills the foreign bacteria. Thus physical and chemical barriers form the first line of defence.

  • Internal Defence : The internal defence is carried on by white blood corpuscles, macrophages, inflammatory reaction, fever and
    • White blood corpuscles (Leucocytes) : The leucocytes in general and lymphocytes in particular are capable of squeezing out through the wall of the blood capillaries into the extra-vascular regions. This phenomenon is called diapedesis. The leucocytes protect in different
  • Lymphocytes : Lymphocytes can produce plasma cells which secrete antibodies to provide
  • Monocytes : They are phagocytic in
  • Eosinophils : Eosinophils can attach themselves to parasitic forms and cause their destruction by liberating lysosomal enzymes on their
  • Neutrophils : They eat harmful germs and are, therefore phagocytic in


  • Macrophages : The macrophages are formed by enlargement of monocytes. They are large cells which are phagocytic in

Mechanism of phagocytes of bacterial cell

performed by a phagocyte


  • Inflammatory Response : When the microorganisms like bacteria, viruses, enter the body tissue through some injury, these produce some toxic substances which kill more cells. These broken cells also release some material which attract the mast cells. The mast cells release histamine. Histamine causes dilation of capillaries and small blood vessels surrounding the injury and increases the permeability of the capillary walls. The more blood flows to area making it red and warm. The fluid (plasma) leaks out into the tissue spaces, causing its swelling. This reaction of the body is known as inflammatory response. The plasma that accumulates at the injured site dilutes the toxins secreted by bacteria and decreases their effect.
  • Fever : The inflammatory response may be in the region of the wound (localized), or it may spread all over the body (systemic). In systemic inflammatory response, the number of WBC increases generally, the fever is caused by the toxins released by the pathogens or by compounds called pyrogens (fever producing substances; Gr. Pre = fire). These compounds are released by W.B.C. in order to regulate temperature of the body. Moderate fever stimulates the phagocytes and inhibits growth of However, a very high fever is dangerous. It is




necessary to bring down fever by giving antipyretics (fever-reducing drug; Gr. Anti = against, pyretos = fever) and by applying cold packs.

  • Interferons : These are the proteins released by the cells in response to a viral infection which they help to combat. These interferons do not inactivate the virus, but they make the unattacked cells less susceptible so they are prevented from the attack of virus. They also prevent the viruses from taking over the cellular machinery. Interferon proteins have proved to be effective in, treating influenza and hepatitis, but their role in cancer treatment is Thus the leucocytes,. Macrophages, inflammatory response, fever and interferons from second line of defence.

Differences between Antibodies and Interferons


Antibodies Interferons
These act inside the cells. They are slow acting.

They act against bacteria and viruses.

Their action is long lasting

These act outside the cells. They are quick acting.

They act only against viruses.

Their action is temporary.

  • Specific Defence Mechanism (The Immune System) : Immune system forms third line of defence. There are two components of immune system in the body : Humoral immune system and cell-mediated immune One of the most important characteristics of the immune system is that it can recognize body’s own cells and macromolecules (self) from those which are foreign invaders (nonself).
    • Humoral Immune System or Antibody-mediated Immune system (AMIS) (Humoral : Pertaining to body fluids): Humoral immune system results in production of These antibodies circulate as soluble proteins in the plasma of blood and lymph which were earlier called humors. The humoral system protects the body against bacteria and viruses that enter the blood and lymph of the body. Antibodies are of many kinds.
    • Cell-mediated Immune System (CMIS) : In this system, highly specialized cells carry out defensive These circulate in the blood and tissue. It protects the body against pathogens including the protists and fungi which have entered the host’s cells. This system also reacts against tissue transplants and perhaps also against the body’s own cells if they become cancerous. Two kinds of cells (T and B cells) are responsible for these responses.

The antigens are foreign ‘molecules’ that invade the body of an organism. The word ‘antigen’ is a shortened form of ‘antibody generating’ because they stimulate the production of antibodies in response to infection. Antigens are generally large molecules. The majority of them are made of proteins or polysaccharides found on the cell walls of bacteria and other cells or on the coats of viruses. All antigens are not the parts of microorganisms. Other structures like pollen grains, white of an egg, shell fish, certain fruits and vegetables, chicken, feathers of birds, blood cells from other persons or animals, drugs, chemicals, etc. can also induce the immune system to produce antibodies.

Classes Description
IgG Main antibody type in circulation; attacks microorganisms and their


IgA Main antibody type in secretions, such as saliva and milk; attacks microorganisms and their toxins.
IgE Antibody responsible for allergic reactions.
IgM Antibody type found in circulation; largest antibody, with 5 subunits,
IgD Antibody type found primarily as a membrane bound immunoglobulin.


Types of Antibodies








Structure of five types of antibodies




(i) Cells of the Immune System : Lymphocytes (a type of WBCS) are the main cells of immune system of the body. Lymphocytes, meant for immune system, are of two types : T-cells and B-cells. Both types of cells develop from the stem cells found in the liver of the foetus and in the bone marrow cells of the adult. Those lymphocytes that migrate to the thymus and differentiate under its influence are called ‘T-cells’, while those cells that continue to be in the bone marrow for differentiation are known as ‘B-cells’. The final maturation of young lymphocytes occur in lymphoid tissues like lymph nodes, spleen and tonsils. T-cells are responsible for cellular immunity, however, B-cells produce the antibodies–about 20 trillions per day that take part in the humoral immunity. Both T-cells and B-cells require antigens to trigger them into action but they respond differently.

B-lymphocytes are independent of the thymus and in man probably complete their early maturation within the bone marrow. They are called B-cells because they mature within the Bursa of Fabricius in birds.

  • Mode of Action of B-Cells to Antigens : When antigens enter a tissue fluid, B-cells are stimulated to produce The body has thousands of antigen-specific B-cells. The membrane of each B-cell type would have been sensitized by the previous contact with the antigen. If this does not happen, the B-cells are destroyed. However, the new B-cells will keep on producing. Once an antigen-specific B-cell is activated by the antigen it multiplies very fast to form a clone of plasma cells. These plasma cells produce antibodies at a rate of about 2,000 molecules per second. This ‘capacity’ of the B-cells to produce specific antibodies is acquired during its process of development and maturation even before it was exposed to an antigen. However, an antigen is necessary to stimulate the production of antibodies.
  • Mode of Action of T-cells to Antigens : Like B-cells, T-cells also respond to antigens by producing a clone (a group) of T-cells. T-cells live for 4-5 years or even longer. There are separate T-cells for each type of antigen that invades the body. T-cells of a clone that are produced in response to an antigen are similar morphologically but they perform different functions. According to their functions, they are of three
    • Killer T-cells : These cells attack directly and destroy In the process, these cells move to the site of invasion and produce chemicals that attract phagocytes and stimulate them so that they can feed more vigorously on antigens. They also produce substances that attract other T-cells.
    • Helper T-cells : These cells stimulate B-cells to produce more of
    • Suppressor T-cells : These cells suppress the entire immune system keeping it away from attacking the own body Some of these cells also become memory cells.

Distribution of B- and T-Cells in Man


Tissue B-Cells % T-Cells %
(1) Blood 15-25% 75-85%
(2) Spleen 55-75% 25-45%
(3) Bone marrow Abundant Few
(4) Thoracic duct 10-20% 80-90%
(5) Lymph nodes 20-30% 60-70%
(6) Thymus gland Few Abundant


Differences between B-Lymphocytes (B-Cells) and T-Lymphocytes (T-Cells)


Feature B-Lymphocytes (B-cells) T-Lymphocytes (T-cells)
(1) Origin and site of differentiation Bone marrow Bursa of Fabricus (in fowl), gut-associated lymphoid tissue (Peyer’s patches) Bone marrow Thymus
(2) Immune System B-cells   form   humoral   or            antibody- mediated immune system (AMIS). T-cells form cell-mediated immune system (CMIS).
(3) Action They defend against viruses and bacteria that enter the blood and lymph. They defend against pathogens including protists and fungi that enter the cells.
(4) Division They are formed by the division of plasma They are formed by the division of

lymphoblasts of three types : killer, helper




  cells. and suppressor cells.
(5) Movement Plasma cells do not move to the site of infection. Lymphoblasts move to the site of infection.
(6) Reaction against Transplants and cancer cells Plasma   cells   do    not   react   against transplants and cancer cells. Killer cells react against transplants and cancer cells.
(7) Effect on Immune System Plasma cells have no inhibitory effect on immune system. Suppressor cells inhibit immune system.


Definition : The resistance of the body to occurrence of any disease is known as immunity. Study of the ability of an organism to resist a disease is called immunology.

  • Development of Immunity : A person may develop immunity in three
  • Vaccination : It is a technique to develop immunity without Weakened or dead pathogens (attenuated) or parts of pathogens are injected into a person who is required to be made immune. The pathogens given in a vaccine are unable to cause the disease but are sufficient to stimulate the formation of antibodies by the host’s immune system. Often 2 or 3 additional doses are needed to generate adequate immunity. These doses are called booster doses.
  • Antitoxins : Antibodies that neutralize toxins produced in the body or introduced from outside are, called antitoxins. Bacterial toxins are produced in the body, however antitoxins produced from outside are prepared from snake venom and is used as a remedy for snake
  • Immunity through Diseases : Some diseases such as mumps, measles, small pox produce a life long Hence these diseases do not appear again.
    • Types of Immunity : There are two main types of immunity : Inborn or innate and acquired or
  • Inborn or Innate Immunity : This type of immunity is inherited by the organisms from their parents and protects it from birth throughout Examples : Human beings have inborn immunity against distemper (a fatal disease of dogs).
  • Acquired or Adaptive Immunity : This immunity is acquired in life time. The acquired immunity is of two types : Active or natural and passive or
    • Active Immunity : When an organism’s own cells produce antibodies it is called active It develops when a person suffers from a disease or gets vaccination for a disease.
    • Passive Immunity : In passive immunity, the antibodies are produced in some other organisms (e.g. vertebrates) in response to the given antigen. These antibodies are then injected into the human body at the time of This is known as inoculation. For example persons infected by rabies, tetanus, Salmonella (causes food poisoning) and snake venom are given the sufficient amount of antibodies so that they can survive.

Passive immunity provides immediate relief, however, active immunity requires some time for the formation of antibodies. There is another form of passive immunity. Nursing mothers transfer antibodies prepared in their body to the infants in their milk. Bottle-fed infants do not get this benefit. After a few weeks, infant’s own immunity system starts working.




Difference between Active Immunity and Passive Immunity

Active Immunity Passive Immunity
(1) It is developed when the person’s own cells produce (1) It is develop when antibodies produced in other
antibodies in response to infection or vaccine. organisms are injected into a person to counter act antigen
  such as snake venom,
(2) It provides relief only after long period. (2) It provides immediate relief.
(3) It has no side effects. (3) It may cause reaction.
(4) It is long lasting. (4) It is not long lasting.

 Disorders of Immune System.

  • Allergies : Allergy is the hypersensitiveness of a person to some foreign substance coming in contact with or entering the

Allergens : The substances that cause allergic reaction are called allergens. The common allergens are dust, pollen mould, spores, fabrics, lipsticks, nail paints, feathers, fur, plants, bacteria, foods, heat, cold, sunlight.

Symptoms : The symptoms that result from an allergy may be of different kinds but mostly it affects the skin and mucous membrane. Hay fever affects the mucous membranes of the nose, eyes and upper respiratory tracts. In asthma, the lower portions of the respiratory system are severely affected. In eczema the skin becomes red, followed by the appearance of minute blisters. Eczema may affect any part of the body and is one of the most severest of all allergic symptoms.

Cause : During allergic reaction there is increased release of histamine from mast cells. It causes marked dilation of all the peripheral blood vessels and the capillaries become highly permeable so that large amounts of fluid leak out from the blood into the tissues.

  • Hay fever : In this allergic form, there is swollen, reddened, running eyes and The drugs called

antihistamines are of major importance in the treatment of this allergic disorder.

  • Asthma : The tissue surrounding the respiratory tubes in the lungs swell up and compress the Hence there is difficulty in breathing. Antihistamine drugs are also given in this disease.
  • Anaphylactic shock : It is an allergic reaction involving all the tissues of the body and occurs in a few minutes after the injection of an antigen such as penicillin. Such a reaction is very Histamine released from ruptured mast cells causes marked dilation of all the arteries so that a large amount of fluid is passed from the blood to the tissues and there is a drastic fall in blood pressure. The affected person may become unconscious and the individual may die within a short time.
  • Autoimmunity : Sometimes it may also happen that the immune system of the body goes off the track and starts behaving against the ‘own body’ or ‘self’. This leads to a variety of diseases known as autoimmune This type of diseases depends on which type of ‘self-antigen’ is involved. When the cells acting as antigens in the same body, they are called autoantigens. The nature of autoimmune diseases depends on the autoantigens involved. For example, if the autoantigens are RBC then the body destroys its own RBC, resulting in chronic anaemia; if the autoantigens are muscle cells then it results in the destruction of its own muscles resulting in severe weakness (myasthenia gravis); if the autoantigens are liver cells, then it results in chronic hepatitis, etc. Other autoimmune diseases are insulin-dependent diabetes, Addison’s disease, ulcerative colitis and rheumatoid arthritis.
  • Immuno deficiencies :
  • Severe Combined Immuno deficiency (SCID) : Sometimes new born children are without T-cells and B-cells. These children are highly susceptible to various The most serious disorder of this type is a congenital disease known as severe combined immuno deficiency (SCID) in which both B-cells and T-cells are not




present in the body. Such children are highly susceptible even to minor infections. In developed countries like

U.S.A. such children are kept alive by keeping them in germ-free environments called isolation suits.

Inflammation : When there is an injury on the skin, the capillaries and small blood vessels dilate to become more permeable to the phagocytes, which leak into the interstitial spaces, engulfing the invading microbes and cleans up the debris. Pus is a collection of dead cells and body fluids. Various chemicals are associated with this type of defence mechanism. e.g. Histamine released by lymphocytes causes the blood vessels to dilate. This inflammatory response may sometimes prevails all over the body i.e. systemic and W.B.Cs, count increases. Generation of heat results in fever which is caused by toxins released by pathogens or compounds known as pyrogens. Such pathogens are released by W.B.Cs. in order to set the body’s thermostate at a high temperature. This stimulates the phagocytes and inhibits the growth of micro-organisms. These barriers are similar for most of the infections. The body backs up this mechanism by more specific immune system.

Autoimmune Disorders

Disorder Symptoms Antibodies Against
Glomerulonephritis Lower back pain Kidney cell antigens that resemble Strep bacteria


Grave disease Restilessness, Weight loss,irritability,

Increased heart rate   and Blood pressure

Thyroid gland antigens near thyroid stimulating hormone receptor, causing overactivity
Juvenile diabetes Thirst, hunger, weakness, emaciation Pancreatic beta cells
Hemolytic anemia Fatigue and weakness Red blood cells
Myasthenia gravis Muscle weakness Receptors for nerve messages on skeletal muscle
Pernicious anemia Fatigue and weakness Binding site for vitamin B on cells lining stomach
Rheumatic fever Weakness, shortness of breath Heart cell antigens that resemble Strep bacteria


Rheumatoid arthritis Joint pain and deformity Cells lining joints
Scleroderma Thick, hard, pigmented skin patches Connective tissue cells
Systemaic lupus erythamtosus Red rash on face, prolonged fever,

weakness, kidney damage

DNA, neurons, blood cells
Ulcerative colitis Lower abdominal pain Colon cells
  • Acquired Immune Deficiency Syndrome (AIDS) : It is a disorder of cell mediated immune system of the There is a reduction in the number of helper T-cells which stimulate antibody production by B-cells. This results in the loss of natural defence against viral infection.

Discovery : AIDS was first noticed in USA homosexuals in 1981. Virus of AIDS was isolated and identified by Prof. Luc Montagnier in France in 1983 and almost the same time by Prof. Robert Gallo in USA. AIDS infections were detected in India for the first time in prostitutes of Chennai in 1986.

In India, four AIDS reference centres have been established.

  • I.I.M.S., New Delhi.
  • National Institute of communicable Diseases, New Delhi,
  • National Institute of Virology,
  • Centre for Advanced Research on Virology, CMC,

AIDS – Related Complex (ARC) : It is a mild form of AIDS. Its symptoms are swollen lymph nodes, fever sweating at night and weight loss. Patients with ARC have a high possibility of early development of AIDS. ARC is also known as a prodromal AIDS.




Universal Immunisation Programme : A programme was launched by WHO in May 1974 to immunise the entire children of the world against six preventable diseases–Diphtheria, tetanus, whooping cough, polio, tuberculosis and measles. The target is to complete the immunisation by the end of year 2000. In India it was launched in 1985 and the target was to be achieved by 1992.

The schedule of vaccination for immunisation is as follows. It is followed by all government hospitals, dispensaries and even private clinics.


S. No. Disease Time Name of the Vaccine
1st Dose 2nd Dose
(1) T.B. Birth to 19 months After 5 years BCG (Bacillus Calmette Guerin)
(2) Measles,                Mumps and Rubella Birth to 12 months Measles vaccine
(3) Polio,            diphtheria whooping cough 3rd, 4th and 5th month Upto 1 1/2 years and again after 5-6 years Polio and DPT vaccine
(4) Cholera Within 2 years Can be given annually before summer Cholera vaccine
(5) Small pox Within 3 months of birth After 3 years Smallpox vaccine
(6) Typhoid 5-6 years, 2 doses with a gap of 1-2 months Booster dose at the age of 10 years Typhoid vaccine


Cells of Immune System


  Cell Type Function
1. Helper T Cell Assists the immune process by helping other cells in the immune system to achieve an efficient immune response.
2. Cytotoxic T Cell Detects and kills infected body cells recruited by helper T cells.
3. Suppressor T Cell Guards against the overproduction of antibodies and overactivity of cytotoxic T cells.
4. Memory cell “Remembers” the original stimulation by the immune system and remains in the lymphoid tissue.
5. Natural killer cell (NK) The lymphocyte without receptor site and help to attack and neutralize virus-infected and tumor cells.
6. B Cell Precursor of plasma cell, specialized to recognize a specific foreign antigen.
7. Plasma cell Biochemical factory devoted to the production of antibodies directed against a specific antigen.
8. Mast cell Initiator of the inflammatory response which aids the arrival of leucocytes at a site of infection, secretes histamine and is important in allergic response.
9. Monocyte Precursor of macrophage.
10. Macrophage The body’s first cellular line of defence; also serves as antigen presenting cell to B and T cells and engulfs antibody covered cells.

Organ Transplants and Immunosuppression : These days, some organs of the body like heart and kidney can be transplanted. Sometimes the organ transplanted is rejected by the recipient body as it can recognise the ‘non-self’ organ and its immune system is stimulated. However, to avoid such rejections drugs called immunosuppressants are used. These drugs are not only expensive but they also make the patient’s body highly susceptible to infectious diseases. So to avoid the use of immunosuppressants, kidney transplants are usually taken




from siblings so that they have a more or less similar genetic constitution. In case of skin grafting, the skin is taken from some other part of the patient body. Now-a-days it has also become possible to take small pieces of skin from the patient’s body and culture it to produce more sheets of skin for transplantation. Radiations also help to suppress the immune system of the body. Efforts are now being made by the scientists to suppress only a small part of the immune system and not the entire immune system of the body.

 Vaccines.                                                                                                                                                                 History of Vaccines and Vaccination : In vaccination weakened or dead pathogens, or portions of pathogens, are injected into a person who is required to be made immune. The pathogens given in a vaccine are

unable to cause the disease, but are sufficient to stimulate the formation of antibodies by host’s cells. The process of

vaccination was initiated by Edward Jenner in 1790. He observed that milkmaids did not contract smallpox apparently because they were exposed to a similar but milder form of disease called cowpox. Edward Jenner infected first James Phipps,, a healthy boy of about 8 years with cowpox and two months later he infected the boy with smallpox. The boy did not suffer from small pox. Jenner proposed that an induced mild form of a disease would protect a person from a virulent form (which has ability to damage the host). He used the term vaccine (in Latin Vacca means ‘cow’) and the term vaccination for protective inoculation. Edward Jenner was the first to discover a safe and effective means of producing artificial immunity against small pox. Thus once vaccination is done the individual is protected from the disease. Vaccination develops acquired immunity. Pasteur confirmed Jenner’s findings and produced vaccines for other diseases like anthrax, rabies and chicken cholera.

  • For protection need : Antibody provoking agents are called These are used against viral and bacterial diseases. Calmette & Guerin developed BCG vaccine for T.B. and Salk made Polio vaccine. Sabin also prepared Polio vaccine. Enders developed vaccine against measles. WHO was formed in 1948 at Geneva to take health problems at global level. In May, 1974, Global Immunisation Programme was launched by WHO for six disease (Diphtheria, Pertussis, Tetanus, Measles, TB & Polio).
  • Vaccination : It is the possible way to induce active acquired immunity against the germs of various diseases such as polio, diphtheria, whooping cough, tetanus and small pox. The immune system is thus induced to produce antibodies against these antigens. The artificial introduction of disease factors in the body is known as Usually 2-3 injections are given to achieve full immunity against a specific pathogen and the further dose is called as booster doses.

Other Vaccines : Vaccines are also available for diphtheria, tetanus, whooping cough, tuberculosis, measles, polio, mumps, plague.

(iii) Types of Vaccines

  • Killed vaccine : These vaccines are prepared by killing the pathogenic organisms by heat uv- rays/alcohol formalin/phenol, g., Typhoid Vaccine, Cholera Vaccine.
  • Toxoid : These are prepared by destroying the toxic property of the toxins produced by organisms but retaining its antigenic property, g., Tetanus toxoid, Antidiphtheria toxoid.
  • Attenuated living vaccines : The pathogen is made weakened to make it nonvirulent, g., Oral Polio Vaccine (OPV), BCG (Bacille Calmette Guerin). MMR (Mumps, Measles, Rubella) Provide active life long immunity.
  • Antibodies as vaccines : Serum is used after a person/animal has been exposed to infection. This serum contains antibodies against that It provides passive artificial immunity for some period only, e.g., ATS (Anti tetanus serum), Antirabies serum.
  • Antigens like polysaccharides of Pneumococci, Interferon (glycoproteins) are also used as





Some Important Vaccines


Name of Vaccine Category of Vaccine Used for treatment of
(1) B.C.G. Live vaccine (actual weakened germs) Tuberculosis
(2) Cholera Vaccine Killed   vaccines  (micro-organisms are killed) Cholera
(3) Mumps Vaccine (MMR) Live vaccine (actual weakened germs) Mumps Measles & Rubella
(4) Oral Polio Vaccine (OPV) Live vaccine Polio, 1st does given when child is 3 months old. Booster does is given after 1 year
(5) Rubella Vaccine Live vaccine German measles and small pox
(6) Rubeolla Vaccine Live vaccine Measles
(7) Tetanustoxoid (TT) Toxoid (bacterial toxin looses toxicity but retains antigenicity) Tetanus
(8) Toxoid Serum Toxoid (bacterial toxin looses toxicity but retains antigenicity) Diphtheria
(9) Typhoid Vaccine (TAB) Killed vaccine (micro organisms are killed) Typhoid (Typhoid & Paratyphoid)
(10) Triple Antigen (DPT) (Diphtheria, Pertussis Tetanus) Toxoid Diphtheria, tetanus and whooping cough, Ist dose given when child is 3 months old. Booster dose at 2 years.