Communicable Diseases

Category : 12th Class

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.

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.

Important diseases caused by Viruses :

(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. Influenza generally lasts for 4 or 5 days. Rest quickens the recovery. 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.

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 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.

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.


Viral Diseases in Humans

S.No. Disease



Main Symptoms Mode of Infection




Myxo viruses

Mucous membrane of respiratory tract

Nasal discharge,  sneezing, coughing

By droplets from nose & throat

24 to 72 hours



Variola virus


Skin rash changing to pustules, then to scabs

By contact, droplets and fomite

12 days


Chicken pox

Varicella zoster


Skin sores that open & emit fluid

By contact and fomite

2 to 5 weeks



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





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


Jaundice due to damaged liver cells

By contaminated food and water

20-35 days


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

(i) 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.

(ii) 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.

Important Diseases caused by Bacteria

(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) : 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 “lock jaw”. 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.

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. Immunity or susceptibility of diptheria is determined by performing the schick test.

(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. 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. Wayson stain test is used for susceptibility of plague.


Bacterial Disease in Human





Main Symptoms

Mode of Infection




Vibrio comma (V.cholerae)


Severe diarrhoea and vomiting

By contaminated food and water

2 to 3 days



Diplococcus pneumoniae


Difficulty in breathing

By patient?s sputum

1 to 3 days



Salmonella typhi


Constant fever

By contaminated food and water

1 to 3 weeks


Tetanus (Lockjaw)

Clostridium tetani


Painful muscular spasms and paralysis

Through wounds and burns

4 days to 3 weeks



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



Mycobacterium tuberculosis


Cough, bloody sputum, chest pain

By patient?s sputum




Pasteurella pestis

Blood and lymph

Painful pubo of lymph nodes

By rat-flea bite

2 to 6 days



Mycobacterium leprae

Skin mucous membranes, peripheral nerves

Hypopigmented skin patches, ulcers, deformity of digits

Long and close contact with patients

2 to 5 years



Treponema pallidium

Oral, genital, rectal mucosa


By contact

3 weeks



Neisseria gonorrhoeae

Urinogenital mucosa

Burning sensation in micturition

By sexual contact

2 to 5 days


Diarrhoeal diseases

Shigella dysenteriae, Salmonella, Escherichia coli, Campylobacter



By contaminated food, water, hands, fomite



(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.

(i) 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”. The patients of syphilis develop characteristic “points” on teeth called ‘Hutchison’s teeth’. Serological tests for early diagnosis of syphilis are TPI (Treponema Pallidum Immobilization test) VDRL (Venereal Disease Research Laboratory test), FAT-ABS (Fluorescent Treponemal Antibody test) and Wassermann test.

(ii) 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.


Sexually Transmitted Diseases (STD) in Human



Causative organism

Nature of Disease

Symptoms - Treatment


AIDS (Acquired Immuno deficiency Syndrome)

Retrovirus - HIV


Enlarged lymph nodes, long fever, weight loss - Nil


Genital Herpes

Herpes simplex virus


Painful ulcer on genitals - Nil


Genital warts

Human papilloma virus (HPVs)


Tumor of the vulva, vagina, anus and penis - Nil



Neisseria gonoerrheae


Infection of all genital organs or PID - Penicillin



Chlamydia trachomatis


White patches on vagina or PID - Nystatin



Treponema pallidum


Cancer and skin eruption - Benzene and Penicillin



Trichomonas vaginalis


Greenish-yellow vaginal discharge-Metronidazole.



Haemophilus ducreyi


Foul discharge and ulcer Drug : Sulphonamide


Lymphogranuloma venerum

Lymphogranuloma psittacosis bacteria


Inguinal lymphadenopathy Drug : Tetracycline


(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.

(12) Anthrax : Anthrax is a common disease of domesticated animals; human may acquire infection through contact with spore-containing animals. Anthrax is caused by a bacterium Bacillus anthracis which produces spores that can remain dormant for many years in the soil. The most common form of anthrax in humans is cutaneous anthrax; other is pulmonary anthrax.

(13) Scarlet Fever : Scarlet fever is caused by the infection of Streptococus pyogenes in upper respiratory tract or pharynx. A toxin-produced rash develops as small “goose pimples” on the skin within 12 to 24 hours. The Dick test is performed to determine the presence of an immunity to scarlet fever.

(14) Botulism (Food poisoning) : Clostridium botulinum is a Gram-positive anaerobic bacillus responsible for food poisoning known as botulism. The bacilli release exotoxin to the environment, which is one of the most potent neurotoxic substance produced by microbes. Main symptomes of botulism are swollen tongue, double vision, vomiting, diarrhoea, fatigue and respiratory faliure.


Insect carrying diseases


Common name

Zoological name

Causative organism




Anopheles sps

Culicine sps

Stegomyia sps

Aedes aegypti


Wuchereria bancrofti

Flavovirus fibricus

Dengue virus




Rat flea

Xenopsilla cheopsis

Xenopsilla sps

Pasteurella pestis

R. typhi

Bubonic plague

Endemic typhus



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


Rickettsia prowazeki

R. Quintana

Trench fever



Trombicula akamushi

R. Tsutsugamushi

Scrub typhus (Tsutsugamushi fever)


Itch mite

Sarcoptes scabieri




Tick fever,

Amblyomma sps

R. rickettsiae

Rocky mountain spotted theileriosis


House fly

Musca domestica

Vibrio cholerae



Infantile diarrhoea


Bed bug



Relapsing fever


Tse-tse fly

Glossina palpalis

Trypanosoma gambiense

Sleeping sickness


Important Diseases Caused by Protozoans :

(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.

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.

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

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 –

(i) 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.

(ii) P. ovale : It also causes benign tertian malaria, which recurs every 48 hours. This species is found only in West Africa and South America.

(iii) 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.

(iv) 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.

(4) 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 cytolysin. This generally results in diarrhoea, but may lead to severe or fatal dysentery.

(5) Trypanosomiasis : It is most serious protozoan disease caused by a flagellate protozoan. Trypanosoma, found firstly in the blood, then in the lymph and finally in the cerebrospinal fluid of man (primary host). Secondary host is a blood sucking insect, Glossina (Tse-tse fly), so the life cycle of Trypanosoma is digentic.


Pathogenic Protozoa


S. No. and name of parasite

Host and site of parasite in its body

Diseases caused

Method of transmission


Class Rhizopoda





ntamoeba 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.


Entamoeba coli

In the colon of man.

Gastro-intestinal disturbances.

By contaminated food and water.


Entamoeba gingivalis

In the buccal cavity of man.

Bleeding gums.

By mouth contact.


Class Mastigophora





Trypanosoma gambiense

In the blood of Africans.

African sleeping sickness.

By the bite of the fly, Glossina palpalis.


Trypanosoma rhodesiense

In the blood of Africans.

Rhodesian sleeping sickness.

By the bite of the fly, Glossina morsitans.


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.


Leishmania donovani

In the liver, lymph glands and leucocytes of man, dog and cat.

Kala-azar fever.

By sand fly, Phlebotomus supp.


Leishmania infantum

In the spleen of children

Enlargement of spleen.

By sand fly, Phlebotomus supp.


Leishmania tropica

In endothelium of blood capillaries of skin of man

Oriental sore.

By sand fly, Phlebotomus supp.


Leishmania brasiliensis

In the infected man, dog and cat.

Skin disease (Espundia in man).

By sand fly, Phlebotomus and contact.


Trichomonas buccalis

In the infected gums of man.

Associated with pyorrhoea.

By infected food.


Trichomonas hominis

In colon of man and other vertebrates.

Associated with dysentry.

By contaminated eatables and water.


Trichomonas vaginalis

In urinogenital tract of women.


During sexual intercourse.


Giardia intestinalis

In small intestine of man


By contaminated food.






        In erythrocytes and liver of man.

Different types of malaria fever.

By the bite of female Anopheles mosquito.





Babesia bigemina

In erythrocytes of cattle.

Taxas fever and diarrhoea.

By the bite of fleas.


Isospora hominis

In small intestine of man.

Diarrhoea and other gastric troubles.

By contaminated food.


Eimeria stiedae

In cells of mucous membrane of hepatic ducts and liver of rabbit.

Diarrhoea and liver disorders.

By their oocysts.


Class Ciliata





Balantidium coli

In colon of human beings.

Ulcers in colon and diarrhoea.

By spores.


Important Diseases Caused by Helminthes

(1) 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 host. This worm has enormous power of reproduction.

(2) Ascariasis : Ascariasis is caused by the roundworm Ascaris lumbricoides. This roundworm lives in the human small intestine. It lies free, having no organs for attachment. It takes host’s digested food by sucking through the mouth (holozoic nutrition). It is more common in the children. The food of the worm consists of semi-digested food of the host, the blood and the fluid of the alimentary canal of the host. There is no secondary host in the life cycle of this parasite. The disease can best be treated by administering antihelminthic drugs such as oil of chenopodium, Alcopar, Bendex, Dewormis, Zental, etc.

(3) Filariasis (Elephantiasis) : Filariasis is caused by the filarial worm, Wuchereria bancrofti. This disease is characterised by the swelling of the legs, scrotum and of some other parts of the body. The disease is, therefore, commonly known as elephantiasis due to its resemblance to a leg of an elephant. The infestation is transmitted by Culex mosquitoes from one dividual to the others. The worms live in the lymphatic system and produce young ones called “microfilaria”. Once the swelling appears, there is no other treatment except surgical operation. A drug, Diethylcarbamazine has been shown to kill the microfilaria.

(4) Ancylostomiasis (Hookworm Disease) : Ancylostomiasis is caused by the hookworm, Ancylostoma duodenale. It lives in the small intestine firmly attached to its wall. 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


Important Helminth Diseases in Humans





Mode of Infection


Taeniasis & Cysticercosis

Taenia solium - the pork tapeworm


By taking raw or undercooked measly pork



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.

Other Topics


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