“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 :

1. Biological Agents : These include viruses, rickettsias, bacteria, fungi, protozoans, helminthes and arthropods. The biological agents are called pathogens (Gr. Pathos = disease; genes = producing).
2. Nutrient Agents : These comprise food components such as carbohydrates, fats, proteins, minerals, vitamins and water.
3. Chemical Agents : These are further of two types

1. Endogenous Chemical Agents : These are formed in the body itself and include hormones, enzymes, urea and uric acid.
2. Exogenous Chemical Agents : These enter the body from outside by inhalation, ingestion or inoculation. Pollutants (fumes, gases, dusts, metals) and allergens (spores, pollen) are examples.

4. Physical Agents : These include heat, cold, humidity, pressure radiation, electricity and sound.
5. Mechanical Agents : These comprise chronic friction or other mechanical forces which result in injury, sprain, dislocation fracture.
6. Deficiency and Excess of substances : e.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

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

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

1. Communicable (Infectious) Diseases : These diseases are caused by viruses, rickettsias, bacteria, fungi, protozoans and worms.
2. Noncommunicable (Noninfectious) Diseases : These diseases remain confined to the person who develops them and do not spread to others. The non-communicable diseases are of four kinds –
   1. Organic or Degenerative Diseases : These diseases are due to malfunctioning of some of the important organs, e.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.
   2. Deficiency Diseases : These diseases are produced by deficiency of nutrients, minerals, vitamins, and hormones, e.g., kwashiorkor, beriberi, goitre, diabetes are just a few from a long list.
   3. Allergies : These diseases are caused when the body, which has become hypersensitive to certain foreign substance, comes in contact with that substance. Hay fever is an allergic disease.
   4. Cancer : This is caused by a uncontrolled growth of certain tissues in the body.

  Communicable Diseases.                                                                                                            

1. 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 –

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

one.

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.

2. A communicable or Infectious Diseases : Caused by pathogens or biological agent. They rapidly spread from one person to another and are of great concern of the society. They are further categorised as :

1. Viral diseases
2. Bacterial diseases
3. Protozoan diseases
4. Fungal diseases
5. Helminthes diseases
6. Sexually transmitted diseases (STD)
7. Diseases through blood transfusion
   3. Control of Communicable Diseases : The Communicable diseases, beside simpering health, have been taking a heavy toll of human life in the past. 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 –

1. To know the nature of the disease, i.e., the causative agent and its life-history.
2. To find out mode of transmission of the disease, i.e., how the causative agents enter the human body.
3. To devise protective measures against the attack of the causative agent of the disease.
   4. Reservoir of Infection for Pathogens : Every pathogen has some reservoir where it normally lives when it is outside the host susceptible to the disease. 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.
   5. Transmission of Diseases (Pathogens) : The diseases (pathogens) are transmitted from the reservoirs of infection to the healthy persons in the following ways –

1. Direct Transmission : The pathogens of some diseases reach the human body directly without intermediate agents. This can occur as under –
   1. 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.
   2. 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 droplets.
   3. Contact with Soil : The bacteria responsible for tetanus and blood poisoning enter the human body from the soil through injuries. Hence, skin injuries should not be neglected.
   4. Animal Bites : Virus of rabies, or hydrophobia, is introduced through the wound caused by the bites of rabid animals, most commonly dogs.
   5. 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 syphilis. The pathogens then pass from the maternal blood into the foetal blood.
2. Indirect Transmission : The pathogens of certain diseases reach the human body through some intermediate agents as explained below –
   1. Arthropod Vectors : Insects transmit diseases in two different ways.
   2. 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 eye. Ants, cockroaches and house crickets also carry disease germs to articles of food.
   3. Certain blood-sucking insects carry disease-causing organisms in their body and transmit them with bites. 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.
   2. Vehicle-borne Method : The causative organisms of dysentery, cholera and typhoid enter the human digestive tract with food, water and ice. 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.

3. Air-borne Method : The pathogens may reach the humans with air and dust. The epidemic typhus spreads by inhalation of dried faeces of infected lice.
4. 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 instruments.
5. Unclean Hands : The unclean hands may carry disease germs to food or mouth. Therefore, hands should be washed before taking meals.
6. 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 mentioned.
   6. Classification of communicable Diseases : The communicable diseases are classified into seven types according to the nature of their causative agent.

1. Viral Diseases : These are caused by viruses. 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.
2. Rickettsial Diseases : These are caused by rickettsias, the obligate intracellular parasitic organisms. They include Rocky Mountain spotted fever, typh’s fever, trench fever and Q fever.
3. Bacterial Diseases : These are caused by bacteria. They include diphtheria, scarlet fever, tetanus, typhoid fever, tuberculosis, anthrax, cholera, food poisoning, and meningitis.
4. Spirochaetal Diseases : These are caused by spirochaetes, the long, spiral, corkscrew-shaped bacteria. They cause syphilis.
5. Protozoan Diseases : These are caused by protists. They include amoebic dysentery, malaria, kala-azar, oriental sore and sleeping sickness.
6. Fungal Diseases : These are caused by fungi, the non-green heterotrophic organisms. They include ringworm and athlete’s foot.
7. 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 roundworms.
   7. How Pathogens Cause Diseases : Pathogens produce diseases in two ways : tissue damage and toxin secretion.

1. 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 haemorrhages. 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.
2. Toxin Secretion : Many microbes produce powerful poisons, called toxins, which cause diseases. Toxins are of 2 types –
   1. Exotoxins : These are released as soon as produced. The diseases brought about by exotoxins include tetanus, scarlet fever, diphtheria, and botulism (food poisoning)
   2. Endotoxins : These are retained in the bacterial cells and released when bacteria die and disintegrate. 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.

1. Diabetes Mellitus

1. Diabetes is characterised by chronic hyperglycemia which is excessive concentration of glucose in the blood.
2. Diabetes is primarily a result of relative or complete lack of insulin secretion by the b cells of islets of Langerhans in pancreas.
3. Diabetes is established by blood and urine sugar levels.

2. Arthritis

1. Arthritis is any inflammatory condition of the joints characterised by pain and swelling.
2. Two kinds of arthritis are : rheumatoid arthritis and osteoarthritis.
3. There is no cure for arthritis; drugs are available which relieve pain.
4. Rheumatoid arthritis is characterised by inflammation of the synovial membrane.
5. A kind of rheumatoid arthritis that occurs in younger people is Still’s disease.
6. Osteoarthritis is a disease common among the elderly persons resulting from erosion of articular cartilage.
7. Paraplegia refer to weakness or paralysis of both legs, often accompanied by loss of sensation.
8. Paraplegia is usually caused by a motor vehicle accident, sports accident, fall or gunshot wounds.

3. Gout

1. Gout results from accumulation of uric acid crystals in the synovial joints.
2. Gout is a disease associated with an inborn error of uric acid metabolism that increases production or interferes with the excretion of uric acid.

4. Cardiovascular Diseases

1. Cardiovascular diseases refer to a number of diseases associated with the blood vascular system.
2. Some major cardiovascular diseases are rheumatic heart disease, hypertensive heart disease and coronary heart disease.
   1. 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.

2. 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.

3. Coronary heart diseases

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

heart.

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

1. Important diseases caused by Viruses : The human diseases caused by viruses include influenza, chickenpox, smallpox measles, rabies, mumps, polio, trachoma, hepatitis and AIDS.
   1. Influenza : Influenza, commonly called flu, is a highly infectious disease, which has still not been conquered. 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.

2. Chickenpox : It is a common, relatively mild, highly contagious disease of children, generally under 10 years of age. 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.
3. Smallpox : Smallpox is an acute, highly communicable disease. 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.

4. 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, i.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.

5. 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 year. 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.

6. Mumps (Infectious Perotitis) : Mumps is an acute communicable disease, generally of children. 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.
7. 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 cord. 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.

8. 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 flies. 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.
9. 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.

10. 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.

2. 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 anthrax.

Bacterial diseases and their pathogens

1. 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 gear. 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.
2. Pneumonia : Pneumonia is a serious disease of the lungs. 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.
3. 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 water. 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.
4. 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 mothers. 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.

5. Diphtheria : Diphtheria is a serious disease of 2-5 years old children. 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.
6. 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 contact. 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.
7. 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 brain. 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.

8. 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. 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.

9. 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 Hansen. 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.
10. Sexually Transmitted Diseases (STD) : The sexually transmitted diseases, also called venereal diseases (VD), spread by sexual intercourse with infected persons. 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.

11. 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

2. Important Diseases Caused by Protozoans : Protozoans cause many diseases in humans. The major ones in our country are amoebiasis, diarrhoea, ciliary dysentery and malaria. Some diseases are given below:

1. 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 humans. 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.

2. 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, i.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.

3. Malaria : Malaria has been for thousands of years a very serious disease of the tropical and temperate regions. 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 –

1. P. 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.
2. P. ovale : It also causes benign tertian malaria, which recurs every 48 hours. This species is found only in West Africa and South America.
3. P. 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.
4. P. 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 world. 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 cytolysi