Communicable (Infectious) Diseases


A communicable disease is one in which the causative organism is carried from one person to another either directly or indirectly. Communicable diseases are classified into following types: 

1): Viral Diseases  : Examples : influenza, small pox, chicken pox, etc. 

2): Rickettsial Diseases: These are caused by rickettsias ( the obligate_ intracellular parasites). The name Rickettsia was given after the name of American Scientist Rickettis (1871__ 1910) .Examples: Rocky Mountain Spotted fever, trench fever, Q fever and typhus fever. 

3):  Diseases caused by Chlamydiae : Chlamydiae occupy a position intermediate between Rickettsiae and virus. They resemble rickettsiae but differ from them significantly by possessing a unique, obligatory intracellular development cycle. Examples: Chlamydia trachomatis stains have been classified by neutralisation and immunofluorescence tests into 15 serotypes __ A,B, Ba, C, D, E, F, G,H, I, J,K,L1,L2 and L3 types , other diseases caused by Chlamydiae are lymphogranulosa, venreum and psittacosis. 

4): Mycoplasmal Diseases: Because  mycoplasmae resemble to the organism causing pleuropneumonia, they were called " pleuropneumonia like organisms " or  PPLO. The causative agent of bovine pleuropneumonia was first isolated by Nocard and Roux ( 1898).Myocoplasmae do not have a rigid cell wall. Parasitic mycoplasmae require cholesterol or other steroids as an essential growth factor.  Mycoplasmae are the smallest free living microorganisms and one of the most pleomorphic with branched, filamentous, breaded and swollen forms.
  (i) Mycoplasma pneumoniae causes pneumonia, tacheobronchitis, myringitis, associated with arthritis. The disease, has an Incubation period of 1__ 3 weeks.

(ii) Mycoplasma hominus is associated with postpartum fever, endometritis, absorson, sterility, septicemia and arthritis. 

5): Bacterial Diseases: Examples : Cholera, typhoid, syphilis, etc.

6): Protozoan Diseases: Examples : Amoebiasis, malaria, kala__ azar, oriental sore, sleeping sickness, etc.

7): Helminth Diseases (flat worms and round worms). Examples: taeniasis, schistosomiasis, liver rot__ caused by flatworms, and ascariasis, filariasis (elephantiasis) enterobiasis, trichinosis, hookworm disease and guinea worm disease caused by roundworms. 

8.Fungal Diseases: These are caused by fungi. Examples : Ringworm, athlete's foot,etc.

☆ Communicable diseases spread from one person to another or from an animal to a person.

☆  The spread often happens via airborne viruses or bacteria, but also through blood or other bodily fluid.

Koch's Postulates 

In 1876, a German microbiologist, Robert Koch discovered that a large number of micro_ organisms such as viruses, bacteria, fungi, protozoans, etc.cause diseases in human body. Koch's postulates  are given below.

1) The organism ( pathogen) must be regularly found in the body of the animal that is suffering from a disease

2) The organism must be isolated that grows in pure culture on artificial media.

3) The same disease must be produced when the cultured organisms are injected into other healthy animals. 

4) The same organism must be recovered from the injected animals. 

 These postulates originally were applied for animal diseases but are equally  applicable for human diseases. However, Koch's Postulates are not applicable to viral diseases because they cannot be cultured on artificial media. Koch's Postulates are also not applicable to bacteria causing leprosy. 

How do these communicable disease spread ? 

Some ways in which communicable diseases spread are by :

■ Physical contact with an infected person, such as through touch ( staphylococcus), sexual intercourse ( gonorrhea, HIV,), faecal / oral transmission  (hepatitis A) or droplets ( influenza, TB).

contact with a contaminated surface or object (Norwalk virus), food ( salmonella, E.coli), blood ( HIV, hepatitis B), or water ( cholera);

■ Bites from insects or animals capable of transmitting the disease ( mosquito: malaria, and yellow fever flea,  plague) and

Travel through the air ,such as tuberculosis or measles 

How to study a communicable disease?

A communicable disease may be studied under the following  headings: 

(i) Causative Organism (= Pathogen)

(ii) Mode of Transmission  ( Mode of infection)

(iii) Causes and Symptoms 

(iv) Preventive measures (Prophylaxis) :

(a) Vaccination if available 

(b) Isolation 

(c) Disinfection 

(d) Sanitation 

(e) Education 

(f) Eradication of vactors.

(A) Important Diseases caused by Viruses 

1)  Common Cold ( Rhinitis) :

It is caused by Rhinovirus ("rhuno" means nose). The virus infects the nose and respiratory passage but not the lungs. Mode of transmission is through inhalation  of droplets resulting from cough or sneezes. It is characterised by nasal congestion  and discharge sore throat, etc.It generally lasts for 3_ 7 days.

2) Influenza  :

It is commonly known as " Flu" and is highly  infectious. The disease is caused by various types of influenza viruses (e.g., Myxovirus influenzae) which are passed out in the discharges from the  mouth and nose of the infected persons ( air borne disease) . It spreads in the same way as common cold. It starts with fever, headache, sore throat, cold with sneezing and pain all over the body with restlessness. It Incubation period is just 24__ 72 hours. Its vaccine  is available. 

3) Small pox ( Variola) : 

It is highly infectious disease starting with fever, chill backaches and headache, followed by appearance of rash on the third day of illness. The rash appear first on the face, then on the rest of the body. It is more on the face and limbs and less on the trunk. The rash starts as small reddish spots which change into papules. These in turn change into small vesicles containing  clear fluid. Vesicles change into postules. Finally,  a scab is formed and it falls off by the third week. These scabs leaves deep pits or scars known as pock marks. This disease is caused by a Variola virus. This virus has a double stranded linear DNA genome with a lipoproteins envelope. Scabe too are infective. Incubation period is about 12 days. Vaccination against small pox is one of the best preventive remedies. This was discovered by Edward Jenner in 1798.

4) Chicken pox ( Varicella) :

It is causing slight fever, and a rash which undergoes changes into vesicles, pustules and finally a dark brown scab which falls off leaving  no scar unlike smallpox. The disease is caused by Varicella zoster which is passed out in the discharges  of the respiratory tract of an infected person directly as droplets or through contaminated articles used by the patient. Its Incubation period is 2__ 3 weeks. So far there is no vaccine  for chicken pox.

5) Measles ( Rubeola) : 

Measles is causing fever, inflammation of the air passage and a rash all over the body. It attacks especially  the children below the age of 5 years, and those who have escaped may be attacked  even in the later  life. It is caused by Rubeola virus which is passed out in the secretions of nose and throat of the infected  persons as droplets or in articles  soiled by these secretions ( air bone disease). Its Incubation period is about 10 days. The eyes are red and watery, and the face is flushed. The rash which  is slightly  pinkish  in colour appears first on the back of the ear and face, and spreads downwards on the body. There may be itching  or burning  sensations along with the rash .Swarts vaccine induces immunity against measles. 

6) Mumps ( infectious  Parotiditis): 

It is an infectious disease causing fever, difficulty in opening the mouth and painful  swelling  of the parotid glands. It is caused by  a Paramyxovirus, which comes out in the saliva of the infected person. Healthy people  get the infection  directly  from the patient  or by coming  in contact with articles soiled with the saliva of  such patients. Its incubation period varies from  12__ 26 days. Usually,  there are no complications, but in some cases there may be pain and swelling of the testes or pain in the abdomen. Mumps is not repeated.Its vaccine  is available. 

7) Polimyelitis or Polio ( Infantile Paralysis) : 

This disease is caused by Enterovirus (Poliovirus). Unit recently, polimyelitis was regarded as disease of infants and children. Hence this  disease was called infantile paralysis. But it is now known that the disease may occur at any age. It spreads through contaminated food or drink and by flies or other insects that may contaminate food or drink. Polio virus usually enters the body via alimentary canal where it multiplies and reaches the nervous system through the blood stream. Its Incubation period is 7__ 14 days. It produces inflammation of the nervous system. The earliest sign of this disease is involvement of the central nervous system causing inability to bend the head forward. Stiffness of the neck is an important sign. Paralysis starts following the weakness of  particular skeletal muscles. Polio vaccine is safe and effective. 

8) Viral Hepatitis:

It is commonly called jaundice. In early stage the liver is enlarged and congested in later stage the liver becomes smaller, yellowish or greenish. The urine is dark and stool is pale, Janundice increases for 1__ 2 weeks. There are 6 varieties of Hepatitis viruses.

(i) Hepatitis  A Virus ( HAV), 

(ii) Hepatitis  B Virus ( HBV),

(iii) Hepatitis  C Virus ( HCV),

(iv)  Hepatitis D Virus also called Hepatitis Delta Virus ( HDV),

(v) Hepatitis  E Virus ( HEV)  and 

(vi) Hepatitis G virus ( HGV) .

There is no Hepatitis  F virus.

Hepatitis A is known as " infectious  Hepatitis ".Hepatitis B, is the most versatile of the hepatotropic virus.Hepatotitis C is clinically  similar to Hepatitis  B, Hepatitis D, called " delta Hepatitis". Hepatitis E virus spread  like Hepatitis A. Vaccine  of hepatitis B is available. 

9) Rabies ( Hydrophobia) :

It is caused by rabies virus.It is introduced in the body by the bite of rabit ( mad) dogs usually. It can be injected by the bite of jackals, wolves, cats etc., Incubation period is commonly 1_ 3 months but may vary from 10 days to one year. Fear of water is  the most important characteristic symptom of this disease. The virus destroys the brain and spinal cord.Rabies is 100% fatal. There should be compulsory immunisation of dogs and cat population. The bitten person should be administered vaccination. The pet should be watched for 10 days after it has bitten  some one to make sure that it does not have rabies virus. Its vaccine  is available. 

10) AIDS_ ( Acquired  Immune Deficiency Syndrome: 

What is AIDS?

It is a disorder of cell mediated immune system of the body. 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 


AIDS was first noticed in USA amongst  homosexuals in 1981. Virus of AIDS was isolated  and identified  by Prof Luc Montagnier in France in 1983 and in 1984 by Prof Robert Gallo in USA .The virus of AIDS was officially named Human Immuno deficiency virus (( HIV) in 1986 by the International  Committee  on Viral  Nomenclature. AIDS infections  were detected  in India for the first time in prostitutes  of Chennai in 1986.

 The AIDS virus may have passed from a monkey host into human population in Africa during 1960s.

 pathogen ( Causative Agent 

The virus was identified  and named by Americans as HCLV III ( Human cell leukemia virus  III), but the name of the virus was changed to HIV (Human immunodeficiency virus) .HIV  is a retrovirus that attacks  helper T cells.

Structure of HIV 

The virus is with a diameter  of about 90__ 120 nm. Its genome consists of two single _ stranded RNA  filaments and associated with a reverse transcriptase enzyme. The envelope consists of a lipid bilayer  formed of glycoprotein. It contains two protein coats.

Mode of Action of AIDS AVirus 

After the entrance of the virus into the body of the person, the virus enters  into macrophages where RNA genome  of the virus replicates to form viral DNA with the help of reverse transcriptase enzyme. This viral DNA gets incorporated into the  host cell's DNA and directs the infected cells to produce  viruses. The macrophages produce virus and act like a HIV factory. Simultan_ eously cells HIV virus enters into helper T lymphocytes where it replicates and produces other viruses. This is repeated  so that the number of T lymphocytes decreases in the body of the infected person. It is due to the fact that HIV destroys T_ lymphocytes. Since the number of helper T lymphocytes decreases in the body, the person starts suffering  from infections  of bacteria especially Mycobacterium, viruses , fungi and even parasites like Toxoplasma .The patient gets  immune deficiency and he/ she is unable to protect himself/ herself against  these infections. 


Aids cannot be acquired by the following

i) Insect bites,

(ii) Crowded Transport, 
(iii) Shaking  hands, 

(iv) Sharing towels,

(v) Coughing and sneezing, 

(vi) Kissing  and embracing, 

(vii) Sharing utilities and telephone,

(viii) Swimming  pools and toilets.

 Could a mosquito  transfer a trace of HIV infected blood that it had picked up to the next person it bites?

This cannot happen. The minimum amount of blood necessary  to transmit  HIV is almost a visible quantity, about a tenth of a millimeter. Given the size of a mosquito's  stinger, it would require  hundreds of mosquitoes biting simultaneously at the spot to transfer  that much tainted  blood. 

Incubation period: The Incubation period of AIDS rages between  6 month  to 10 years.

 Incubation period 


Diagnosis Treatment 

Prevention ( Prophylaxis) 


AIDS _ Related  Complex ( ARC)

11) Dengue Fever ( Break__ bone fever) :

It is caused by an RNA containing  arbo virus ( Arthropod born virus) of flavi virus group which also causes  yellow fever ( not found in India). Thus, the virus which causes dengue fever is a mosquito borne flavi__ ribo virus. The virus of dengue fever is transmitted  by the bite of Aedes aegypti, tiger mosquito. Incubation period is 3__ 8 days. Two types of dengue fever are found : classical  dengue fever and dengue haemorrhagic fever.

(a) Symptoms  of Classical Dengue Fever :

(i) Abrupt onset of high fever .

(ii) Several frontal headache. 

(iii) Pain behind the eyes which worsens with eye movement.

(iv) Muscles and joint pains.

(v) Loss of sense of taste and appetite.

(vi) Measles  like rash over chest and upper limbs.

(v) Nausea and vomiting. 

(b) Symptoms  of Dengue  Haemorrhagic fever : 

Symptoms  similar to classical dengue fever except  the following  :
(i) Bleeding  from the nose, mouth, gums and skin bruising.

(ii) Severe and continuous stomach  pains.

(iii) Frequent vomiting  with or without  blood.

(iv) Pale cold or clammy skin.

(v) Excessive thirst ( dry mouth).

(vi) Rapid weak pulse.

(vii) Restlessness and constant  crying. 

Treatment  :

If there is fever consult the doctor  at once, take paracetamol tablets on  the advice of doctor. Do not take Aspirin  and Dispirin.Rush the patient  to the hospital  if there is bleeding from  any part of the body or of the patient  becomes  unconscious. 


No vaccine  for Dengue fever is available. Eliminate mosquito breeding places by covering small water containers, water tanks, changing the water of cooler every week, and where Aedes mosquito breed. Wear clothes  which cover arms and legs. Do not allow children  to play in shorts and half sleeved clothes. 

12) Yellow Fever  : 

It is caused by an arbovirus. It is a haemorrhagic disease transmitted by the infected Aedes aegypti mosquito.Its symptoms  are fever, rupture of veins in kidneys spleen, liver, etc. In severe cases, the skin of the patient  becomes yellow  from jaundice, hence its name .Max Theiller got Nobel prize  for the development  of vaccine  for yellow  fever  in 1951.

13) Chikungunya :

It is caused by Chikungun virus. This virus was first isolated from human patients and Aedes aegypti mosquitoes from Tanzania in 1952. The name ' Chikungunya ' is derived from the native word for the disease in which patient lies " doubled up" due to severe joint.In 1958, the virus caused a large epidemic of haemorrhagic fever in Thailand. The virus appeared in India in 1963 when alongwith  dengue it caused  very extensive epidemics in Kolkata, Chennai and other areas. Chikungunya outbreaks have occurred  at irregular  intervals along the east coast of India and Maharashtra till 1973. Its symptons include sudden onset of fever, crippling joint pains, lymphodenopathy and conjunctivitis.  Some show haemorrhagic manifestations. The fever is typically  biphasic with  a period remission  after 5__ 7 days  of fever. The vector is edes aegypti. No vaccine  is available. Some cases of Chikungunya  have been reported  from India in 2006.

B) Important  Diseases  caused by Bacteria:

1) Cholera :

This is caused by Vibrio cholerae.Robert Koch first isolated  Vibrio cholerae in 1883. The stools and the vomit of a cholera patient  are full of cholera germs and these may get into a healthy  person when contaminated food or drink is taken by mouth. Incubation  period varies from a few hours to 2_ 3 days. The patient  starts passing  stools frequently, which are white like rice water, and gets repeated vomiting. Rapid replacement of fluid and electrolytes is needed by oral rehydration therapy. Cholera vaccine  is available. 

2) Typhoid ( Enteric fever) : 

This is caused by Salmonella typhi. The Incubation  period of the disease is 1_ 3 weeks. The organisms of the disease  are present  in the stool. They may be present  in urine. They can, therefore, be carried by water and contaminated  food. Incubation  period varies from 1_ 3 weeks ( average 2 weeks). The temperature goes on rising during the first  week and then gradually  comes down. Relapse is common. Since the disease  spreads through contaminated  water and food, all measures for disinfecting water supplies and preventing contamination of food by flies  and dust are to be adopted . Typhoid vaccine  is available . A test to confirm  typhoid is called Widal Test.

3) Diphtheria :

This disease  is caused by Corynebacterium diphtheriae usually  affecting children  upto five years of age. Throat and or tonsils show a grey membrane  which may spread  down and cause hoarseness and difficulty in breathing. Nose may be affected  giving  rise to a blood__ tinged nasal discharge from one nostril. If the disease  is not treated  early and properly  the toxin produced by the germs affects the heart  and the nervous system, and proves fatal. The germs are present in the discharges from the nose and throat of patients  and also of healthy  people who act as the " carriers". The patients and the carriers spreads  the disease through acts like Kissing , talking , coughing  and sneezing. The clothing, utensils , toys, pencils, etc.may be contaminated.The germs may also be transmitted  in milk. Incubation period is 2_ 5 days. Immunity or susceptibility of diphtheria is determined by the Schick test.The most important  preventive measure __ against  this disease is that all babies  should be, immunised within the first six weeks of birth using DPT vaccine  ( Diphtheriae Pertussis  Tetani Vaccine).

4) Whooping Cough ( Pertussis) : 

It is caused by Bordetella pertussis which comes  out while coughing  from the discharges  of the nose and throat of the patient. It spreads by direct inhalation of droplets from the patient  or the carrier, or by the articles freshly soiled by the discharges. Incubation  period is 10__ 16 days. Whooping  sound is produced. The disease can be prevented  by immunising all infants with DPT vaccine. 

5) Tetanus ( Lock Jaw ) :

It is caused by Clostridium  tetani.Spasms of muscles of the jaw and face take place and thus " Lock Jaw" occurs. It is often a fatal disease. Tetanus organisms live in the intestine of horses and other animals without doing any harm. The spores, are therefore,  abundant in the with animal dung. Spores may survive for 60 or more years in contaminated soil.On entering the body by way of wounds, the spores release  active bacteria. The latter multiply and secrete powerful toxin into the tissue  and blood. Incubation period varies from 4 days to 3 weeks. All wounds should be treated  carefully  and cleaned with iodine solution. Tetanus toxoid injection  should be administered in case  of an injury. This will prevent  the tetanus.It is now a common  practice  to immunize the infants against  tetanus, diphtheria and whooping  cough . DPT vaccine  is used.

6) Pneumonia:

This disease is caused by Diplococcus pneumoniae. Pneumonia is a disease of the lungs with an Incubation period of 1_ 3 days.Lymp and mucus collect in the alveoli and bronchioles of the lungs so that the lungs do not get sufficient air. Therefore, proper exchange of gases does  not take place in the alveoli. It  usually  lowers bbody resistance. Infection  spreads by sputum of the patient. 

7) Plague : 

This disease is characterized by a bubo in the groin or the armpit. Plague is caused by Yersinia pestis. It goes from rat to rat through the rat fleas. But when the  rats die of plague, the fleas leave the dead rat and if any man is round about, they bite him and accidentaly inject into his blood some plague germs. In its typical form the bubonic plague is not transmitted from one man to the other, but always from a rat to one or more men.The incubation period  of bubonic plague is 2_ 6 days. Watson stain test is useful for susceptibility of plague. The control measures include the control of rat population, use of insecticides to destroy fleas and sleeping  on a high  cot as fleas cannot jump more than 45 cm or so. Inoculation of anti__ plague vaccine is very use for individual protection against plague. Plague reached India in 1895 with ships from Hong Kong.

8) Tuberculosis ( TB or Koch's  Disease) : 

It is also called the " Great White Plague". It is caused by Mycobacterium tuberculosis. The bacteria damage  the tissues and release a toxin named tuberculin which produces the disease. It affects the lungs, lymph nodes,bones and joints.Incubation period is quite variable. Symptoms of pulmonary (lungs) tuberculosis are fever, cough, blood containing sputum, pain in the chest and loss of weight, excessive fatigue, failure of appetite, slight rise of temperature in the evening, hoarseness of throat, night sweating and rapid pulse.Modes of infection includes infection by inhalation of droplets expelled by tubercular patients, infection of food and drink contaminated with bacteria of tuberculosis, milk from a tubercular cow, etc.BCG ( Bacilus Calmette Guerin) named after their discoveres ( Calmette and Guerin) vaccine  gives considerable protection  against tuberculosis. Tuberculosis is cured by rifamin world T.B. Day is celebrated on 24th March.

9) Leprosy  ( Hansen's  Disease) :

This disease is caused by Mycobacterium leprae,which was discovered  by Hansen.  
Symptoms of leprosy include appearance of light coloured patches on the skin, partial or total loss of sensation  in the affected  parts of the body. These are accompanied  by fever, pain, ulcers and skin eruptions.Difomities of toes and fingers may also develop. 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. Leprosy is curable. The patient  is treated  with DDS ( diamino diphenyl sulphone). 30th January  is celebrated as " Antileprosy Day".

10) Diarroeal Diseases 

These are a group of intestinal infections, including food poisoning. The main symptom of it is diarrhoea. Infections spread through water, food, drinks, fingers, clothes, bed sheets and utensils. The causative agents  are  mainly  bacteria such as Escherichia coli,Shigella dysenteriae, Campylobacter and Salmonella. Giardia intestinalis ( a protozoan) and certain viruses can cause diarrhoea.Toxins released 
by E.coil cause mild diarrhoea to severe dehydration. Symptoms are frequent passage of stools and abdominal cramps. Incubation period  is available.All diarrhoea diseases  cause dehydration. Therefore, in such conditions, patients should intake adequate fluid and electrolytes. This is called oral rehydration therapy. 

11) Gonorrhoea : 

It is a sexually transmitted  disease ( STD), also called veneral disease ( VD). It is spread by sexual intercourse  with infected person.It is caused by  Neisseria gonorrhoeae. Neisser isolated Neisseria gonorrhoeae in 1879. Gonorrhoea may be asymptomatic and go undetected until complications appear. When there are symptoms, a discharge of pus  from the penis or excessive vaginal secretion may be seen. However, signs are less apparent in women than in men, and vaginal  secretion may be ignored as only slightly greater than the normal. Burning on urination and pain around the genital is also  frequent symptoms. Incubation period is 2 to 14 days in males and 7 to 21  days in females Gonorrhoeae is curable with antibiotics. 

12) Syphilis:

It is also a STD or VD  which is also spread by sexual intercourse with infected person. In 1905 Schaudinn and Hoffmann discovered that syphilis was  caused  by a spirochaete ( spiral bacterium) which they  called at first, Spirochaeta pallida. Later they changed  the name to Treponema  pallidium, but because  the first name had widely accepted, both are now regarded as correct. Incubation period is about 3_ 4 weeks. The symptoms  of   syphilis occur in three stages. The first stage usually  consists of a painless lesion called  a chancre at the organism's site of entry. The chancre is usually  on external genitals, but in women it may be inside the vagina The chancre heals quickly even without  treatment, and the victim is likely to think that the disease is gone. The second stage begins as the  organism enters the blood. Symptoms such as fever, a flu like illness, a skin rash, hair loss, and swollen joints may come and go  over a period of several  years. In the third stage permanent brain damage, heart disease, and blindness often occur. Syphilis can be cured with large doses of antibiotics. Unlike gonorrhea, syphilis can be transmitted across the placenta to a foetus. The organisms enter the foetus's blood stream  and invade all the tissues of the body. This disease, congenital syphilis,may cause brain damage and even death  of the foetus, or the child can be born and die later. 

13) Anthrax :

It is  caused by Bacillus anthracis,Infected animals shed in the  discharges from  the mouth, nose and rectum a large number of bacill ( bacteria) which spoculate in the soil. These spores are source  of infection. Human anthrax is contracted from animals  directly  or indirectly. The disease may be 

(i) Cutaneous 

(ii) Pulmonary  or 

(iii) gastro__ intestinal .

   Cutaneous anthrax follows entry of infection through the skin. The face, neck, hands,  arms and back are the usual sites. The lesion starts as a papule 1__ 3 days after infection  and becomes vesicular, containing fluid.The disease is common in dock workers carrying loads of hides and skin on their bareback. Cutaneous tissue anthrax is also caused by shaving  brushes made with animal hair.  Pulmonary anthrax is common in workers in wool factories , due to inhalation of dust from infected wool. This is a haemorrhagic Pneumonia. Gastrointestinal anthrax is rare and occurs mainly in those persons who eat the carcases of animals dying of anthrax. In the patient bloody diarrhoea occurs. It is found in the workers of meat packing  or wool factories. It is also found in butchers and farmers. 

C) Human Diseases caused by Chlmydiae 

D) Rickettsial Diseases  of Man

 E) Important  Diseases caused by Protozoans 

1) Amoebiasis ( = Amoebic Dysentery) :

This disease is caused by Entamoeba histoloytica. There is one host,i.e., man. The parasite lives in the large intestine. Infection  occurs by ingesting cyste with food and water.

Lamble ( 1859) discovered Entamoeba histoloytica. Losch ( 1875)  discovered its pathogenic nature. It is monogenetic (single host life cycle) more commonly found in male than in females. Probably the internal epithelium of the large intestine ( more common in colon) gets dissolved by a proteolytic enzyme resulting the exposure of blood vessels. The it eats red blood corpuscles and ruptures cells of the host intestine through  holozoic mode of nutrition. 

In certain cases, E.histolytica enters the blood circulation of the host through which it reaches its liver, lungs spleen or brain where it causes fatal results. 

  E.histolytica occurs in two forms : magna and minute forms :
(i) The  magna form responsible for the destruction of the protoplasm and usually red blood corpuscles. They have single pseudopodium. 

(ii) The minute forms are smaller and lack pseudopodia and food  vacuole but take nourishment from dissolved substances. Sometimes, bacteria are also eaten, they are harmless .They also undergo cyst formation which are mainly  used for the transmission of  the parasite from one host  to the other. The encysted minuta forms are passed  out alongwith the faeces of the host . These cysts may be flown by  the air and get deposited on vegetables, fruits, etc. When a person eats these edible things the cyst will be transferred into the digestive tract. The cyst ruptures under the influence of the gastric fluid secreted in the stomach of the host. Thus active parasites are  liberated from the cyst into the intestine where they start normal life. In amoebic dysentry the patient passes blood along with the faeces, and feels pain in the abdomen. The magna forms feed upon the  red blood corpuscles, and other useful body substances, so that the patient becomes weak. 

2) Giardiasis (= Diarrhoea) :

It is caused by a zooflagellate protozoan named Giardia intestinalis. Giardia was discovered by Leeuwenhoek in his own stools in 1681.It is the first human  parasitic protozoan known. It lives in the upper parts (duodenum and jejunum) of human small intestine.It is found all over the world. It absorbs nourishment from the food passing through intestine, frow and multiply through binary fission.The large number of parasites interfere with digestion and absorption of food. This causes epigastric plan, abdominal discomfort, diarrhoea, headache and sometimes fever. The disease caused by Giardia is popularly known as giardiasis or diarrhoea ( watery and frequent stools). 

3) Trypanosomiasis ( = African Sleeping Sickness) : 

This disease is caused by  zooflagellate protozoans Trpanosoma gambiense. It is common in western and central parts of Africa.The parasite is transmitted by blood sucking tse_ tse fly, Glossina palpalis. The reserve host is antelope . The parasite does not affect antelope and the fly.In human beings the parasite  lives in the blood plasma.It produces trypanosoma fever.It is accompanied  by glandular swelling . Later the parasite enters the cerebrospinal fluid damages the  brain. It makes the patient lethargic and unconscious. Because  of it, the disease is called sleeping sickness. 

4): South American Trypanosomiasis ( = Chaga's Disease): 

It is caused by Trypanosoma cruzi.Man ( secondary host) is infected either by faecal matter of reduviid bug being into the wound caused by the bite or by a possible  contamination of the conjunctivae. Transmission also occurs via placenta to newborns, and blood transfusions. The entry is through skin or conjunctivae.Two forms of Chaga's  disease are found :

(i)  Actue form : It  occurs in children and infants and is characterised by fever, conjunctivitis, unilateral oedema of the face, enlargement of the spleen and lymph nodes, anaemia and lymphocytosis ( increase in the number of lymphocytes). It lasts for 20 to 30 days. It often becomes fatal involving brain and heart.

(ii) Chronic form : It is found in adults and  adolescents and is characterised by disturbances of cardiac rhythm and neurological problems. It may last for 12 years.

5) Malaria Cause :

Malaria is caused  by the malarial parasite ( = Plasmodium) which has two hosts :

(a) Female Anopheleles Mosquito : As the sexual phase of the malarial parasite occurs in the mosquito it is considered the definitive ( = primary) host of malarial parasite. 

(b) Human beings: As the asexual phase of the  malarial  paraoccurs in man, it is considered the intermediate ( = secondary) host.

 As the female Anopheles mosquitoes feed on blood, only they can serve as vector hosts (= carrier) of malarial parasites. The parasite does not harm the mosquito. 

Historical Aspect : The name malaria was given by Muculoch in 1872 on the belief that it was caused by the foul air ( Italian mala_ bad, aria_ air). Lancisi ( 1717) first suspected a relationship between  swamp, malaria and mosquito. Laveran ( 1880) discovered that malaria is caused  by protozoan parasite. In fact he discovered Plasmodium.He got Nobel Prize in 1907.His topic of discovery was " Role of Protozoans in Causing Disease". Golgi ( 1885) confirmed Laveran's discovery by observing stages of Plasmodium malariae in human RBCs. In 1897 Sir Ronald Ross, a doctor in Indian Army, established that malarial parasite is transmitted by the bite of a female of a female Anopheles mosquito.
In 1902, he got Nobel Prize  for this discovery. He worked in Secunderabad, India. Romanowsky ( 1891) in Russia developed a method of staining  malarial parasite. Grassi
(1900) described the life cycle of Plasmodium
in the stomach  of female  Anopheles mosquito. Shortt ( 1948) and Garnham  (1954)  discovered the pre_ erythrocytic schizogony  (Plasmodium  in human liver). Dr. Manuel Patarroyo produced synthetic vaccine against malaria in 1987. It proved effective in 60  to 70% of cases.

Human Species of Plasmodium and Types of Malaria :

In human beings malaria is caused by four species. 

(i) Plasmodium vivax : It is most common in India. Its Incubation period is about 14 days. It causes Benign Tertian  Malaria. Recurrence of fever is after every 48 hours ( every third day). Recurrent attacks of fever is called paroxysms. Early trophozites with Leishman's stain appear to consist of a blue cytoplasmic ring, Schuffner's dots.

(ii) Plasmodium falciparum : It is common in certain parts of India. It is the greatest  killer of human beings over most parts of Africa and else where in tropics. Its Incubation period is about 12 days. Recurrence of fever is after every 48 hours ( every third day). It causes Malignant (= Ae_
stivo__ autumnal or pernicious or Cerebral or Tropical) Tertian Malaria. Murer's dots or clefts ( staining bricked with Leishman stain).

(iii) Plasmodium malarial :It is common in tropical Africa, Burma, Sri Lanka and parts of India. It is less common  in India.This was the species of malarial parasite discovered by Laveran. This is the only species which can also infect other primates. Its Incubation period in 28 days.Recurrence of fever is after 72 hours ( every 4th day).It  causes Quartan Malaria. After prolonged staining, Ziemann's dots are seen.

(iv) Plasmodium Ovale :This is the rarest of the four species which infect man. It is mostly  found in tropical Africa.Its Incubation period is about 14 days.It causes Mild Tertian Malaria. Infected RBCs show granules like Schuffner's dots and take a violet tinge. These are named Jame's  dots. 

In addition to above mentioned malaria there are two more types of malaria :

(a) Quotidian Malaria: Paroxysms are irregular and almost daily. It may result  from secondary complication of malignant tertian or due to multiple or mixed infections by more than one species of Plasmodium.

(b) Relapse Malaria  : In P. vivax, P. malariae and P. Ovale, exoerythrocytic cycles continue uninterrupted even after the disease is completely cured. Merozoites ofthese Cycles can anytime attack RBCs and cause a relapse
of malaria.

Symptoms of Malaria:

The patient  displays symptoms of malaria fever after a period of 14 days from infectious bitess.Early restlessness, less appetite and slight sleeplessness and followed by muscular pains,headache and a feeling of chilliness. In response to chills the body temperature starts rising and may reach 106°F at the height  of fever . The patient sweats a lot and the temperature  steadily goes down to normal, till the next attack takes place after 48 hours.

Incubation Period : As mentioned above, the symptoms of malaria first appear about 14 days after the infection of the malaria parasite (P.vivax) in man. This interval of time is known as incubation period. It is 14 days in P.vivax. During this period the parasites multiply to increase their number so that they can produce enough toxins to cause malaria. 

Control of Malaria : Malaria  is widely spread disease in India. There is separate anti__ malaria department of the government which controls malaria through National Malaria Eradication Programme ( NMEP).

      Quinine, the oldest drug for malaria, and other drugs are also used for this purpose. Quinine extracted from  the bark of the cinchona tree which is mostly growing in West Indies, India, Sri Lanka, Java and Peru. Other anti__ malaria drugs are paludrine  and  Primaquin, chloroquinine, camoquin  and comoprima. Mosquito nets should be used. The exposed body parts maybe protected by using mosquito repellant  creams, oil, etc. The persons living in malarious regions are advised to take small regular dose of preventive medicines such as quinine, paludrine which kill the plasmodia in sporozoite or merozoite stage. Mosquitoes and their larvae and pupae should be killed by insecticides. Ducks, larvivorous fish like Gambusia, some adult insects like dragon flies, insectivorous plants such as Utricularia, are the natural enemies of mosquito larvae and pupae as they feed upon them. These may be introduced in the water containing the larvae and pupae.

6) Kala__ azar ( Dum__ Dum Fever) :

It is caused by Leishmania donovani.The parasite is transmitted by Phlebotomus argentines (sandfly) .Its symptoms are continuous fever, anaemia, enlargement of liver,spleen, etc.

7) Trichomoniasis:

It is caused by Trichomonas vaginalis. It lives in the vagina of women. The symptoms of this diseases are burning  sensation, itching and frothy discharge. In males the parasite produces irritation in urethra.Its transmission is through sexual act.

8) Balantidiosis (= Balantidium Dysentery) : 

It is caused by Balantidium coli. This parasite lives in the human large intestine ( colon). It feeds on human red blood corpuscles, tissue fragments, undigested food and bacteria. It also undergoes cyst formation. Cysts are passed out in the host's  faeces. Infection occurs by ingesting cysts with food and water. Balantidium coli invades mucous membrane of the colon by secreting an enzyme hyaluronidase. The parasite causes ulcers in the human colon and diarrhoea but may also lead to severe dysentery.  Ciliary dysentery can be prevented by protecting food articles from dust and filies that carry cysts of Balantidium coli.

Susceptibility to infection

Pandemics such as COVID _ 19show that people dramatically differ in their susceptibility to infection. This may be because of general health, age, or their immune status, e.g. when they have been infected previously. However, it also has become clear that there are genetic factor which determine susceptibility to infection. For instance, up to 40% of SARS_ COV_ 2 infections may be asymptomatic, suggesting that many people are naturally protected from disease.Large genetic studies have defined risk factors for severe SARS-CoV-2 infections, and genome sequences from 659 patients with severe COVID-19 revealed genetic variants that appear to be associated with life-threatening disease. One gene identified in these studies is type I  interferon  (IFN). Autoantibodies against type I IFNs were found in up to 13.7% of patients with life-threatening COVID-19, indicating that a complex interaction between genetics and the  immune system is important for natural resistance to Covid.

Similarly, mutations in the  ERAP2gene, encoding endoplasmic reticulum aminopeptidase 2, seem to increase the susceptibility to the plague, the disease caused by an infection with the bacteria yersinia pestis. People who inherited two copies of a complete variant of the gene were twice as likely to have survived the plague as those who inherited two copies of a truncated variant.

Susceptibility also determined the epidemiology of infection, given that different populations have different genetic and environmental conditions that affect infections.

F) Important Diseases caused by Helminths 

1) Taeniasis :

Taeniasis is caused by Tanenia sodium. It is found in all those countries, where pork is taken as food. The adults of T. Solium are parasites in the small intestine of human beings (primary host) and its larva infests  the muscles of the pig mostly (secondary  host). Other vertebrates like ox, monkey, sheep, dog, even man may also serve its secondary hosts. Taeniasis can be recognised by such symptoms as abdominal pain, restlessness, anaemia, insomnia, and loss of appetite ( anorexia) coupled with indigestion. The conventional antihelminth medicines like Carbon tetrachloride, Antiphen, Camoquin prove effective. The pork should be fully cooked before eating and the faecal matter of infected human beings should be destroyed in pits.

2) Schistosomiasis (= Bilharzia) :

It is caused by a flatworm Schistosoma (=Bi_
harzia). Schistosoma shows remarkable sexual dimorphism. Male has gynaecophor_
ic canal in which the female resides permanently. Man is primary host and snail is secondary host. Schistosoma (=Biharzia) 
feeds on human blood.

3)  Fasciolopsiasis :

It is caused by Fasciolopis buski.Its primary hosts are man and pig. It lives in small intestine mainly duodenum and hejunum. Its secondary host is aquatic  snail of the genus segmentina.Mode of infection to man is by eating infected plants as raw food stuff.
The symptoms of Fasciolopsiasis are asthen_
ia, chronic diarrhoea, oedema and eosinophilic. 

4) Ascariasis :

It is caused by Ascaris lumbricoides.It is an endoparasite of the small intestine (mainly     jejunum and  ileum) of human beings, but also infects the pigs and cattle.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. Since a large number of adult Ascaris worms normally infest a  single host, they obstruct the intestinal passage and thereby cause abdominal discomforts, like colic pains. The patient may also suffer from impaired digestion, diarrohea and vomiting. The disease can best be treated by administering antihelminthic drugs such as oil of Chenopodium , Alcopar, Bendex, 
Dewormix, Zental,etc.

5) Filariasis :

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. Filariasis is caused by a number of worms. But in India only two types of worms are responsible, and are called Wuchereria Bancrofti and W. malayi. The infestation is transmitted by culex mosquitoes from one individual to the others.The worms live in the lymphatic system and produce young ones called " microfilaria". When a mosquito sucks blood from an infected individual, the microfilaria which are harmless to man enter the stomach of the mosquito. These microfilaria develop and grow to form infective larvae which now come to lie on the mouth parts of the mosquito. When an infected mosquito bites any person, the larvae are deposited on the skin from where  they make their way into the skin swell. Eventually they enter the lymphatic system where they  develop into adults. In human beings, the Incubation period is 5 to 18 months. Once the swelling  appears, there is no other treatment except surgical operation
.A  drug, Diethylcarbamazine has been shown to kill the microfilaria.

6)  Ancylostomiasis ( Hookworm Disease) :

It is caused by Ancylostoma duodenale (hookworm). The hookworm is found as an endoparasite in the small intestine ( jejunum, less often in the duodenum,  rarely 
in the ileum) of human beings. The worm causes acute anaemia due to extensive injury to the host intestinal wall. Thus it causes mental and physical deficiency.  Its infection is more prevalent in rural areas where people move bare__ footed and hygiene is neglected. 

7) Enterobiasis ( Oxyuriasis ) :

This disease is caused by Enterobius vermicularis (pinworm). It is found in human caecum, colon or vermiform appendix. It is more common in children. Fertilized females make trips to the anus of the host to lay eggs. re deposited on the perianal region. The patient scratches the affected area. The eggs easily get under the finger nails from where they may get into the mouth. The eggs may infect the other members of the family through clothes, dust, air and furniture in the room. The worm causes intense itching of the anus,inflammation of mucous membrane of color and appendix, and nausea, abdominal pain and diarrhoea. Its life cycle is monogenetic.

8) Trichinosis :

It is caused by Trichinella spirals which lives as an endoparasite in human  small intestine ( in the duodenal or jejunum mucosa). The symptoms of the disease are diarrhoea, nausea (feeling of vomiting), abdominal pain, muscular pain, etc. Man usually  receives infection by eating  partly cooked  pork containing very  minute cysts.

9) Dracunculiasis ( Guinea Worm Disease) :

It is caused  by Dracunculus medinensis ( guinea worm) that occurs in the sub _ cutaneous tissue  of man. The parasite produces  toxic secretions which cause a blister on the skin of the host . When the host comes in contract with cold water, the female worm found below the skin, releases in the water. The larvae penetrate the body of cyclops, a fresh water crustacean, which is the intermediate  host  of the guinea worm. When the infected cyclops is taken in by man with water, the larvae escape from the cyclops into the human intestine. The larvae bore their way to the subcutaneous tissue where they become adults. The guinea worm causes itching, diarrhoea, nausea, vomiting and eosinophilia due to its toxic  secretions in the human body.

10) Whipworm Disease:

It is caused by Trichuris trichiura (Whipworm). It lives in the large intestine particularly the carcum. Itis also found in the vermiform appendix, and colon. New host gets infection by ingesting  the embryonated eggs with contaminated food or water. It causes abdominal pain, diarrhoea and anaemia. 

11) Loiasis ( Eyeworm Disease) :

It is caused by Loa loa ( African eye worm). Adult worm lives in subcutaneous tissue of man  often in the sub_ conjunctival tissue of the eye. Man is infected by the bites of Chrysops (deer fly). The symptoms of loiasis are oedema ( swelling) in eye and allergy. 

G) Fungal Disease:

The dermatophytes are a group of closely related filamentous fungi. They infect only superficial keratinised tissues such as skin, hair and nails. They cause a variety of clinical conditions, collectively called as dermatophytoses ,popularly called tinea and ringworm.
      Dermatophytes include three genera :
☆ Trichophyton : It infects skin, hair and nails. 

Microsporum : It attacks the hair and skin but usually  not the nails.

Epidermophyton : It infects the skin and nails but not the hair. Thus main symptons of the disease are appearance of dry, scaly lesions on various parts of the body such as skin, nails and scalp. These lesions are accompanied by intense itching. 

Some diseases  of humans caused by fungi are as follows :

(i) Tinea pedis  or " athlete's foot"  is ringworm of the food which is caused by Trichophyton rubrum and Epidermophyton floccosum.

(ii) Tinea capitis is ringworm of the scalp ( skin) and hair of the head, caused by Microsporum, any species, and Trichophyton, most species. 

(iii) Tinea  cruris   involves the groin and the perineum. It is caused by E. floccosum and T. rubrum 

(iv) Tinea barbae or barber's itch involves the bearded areas of the face and neck.It is caused by T. rubrum, T. mentagrophytes,T. verrucosum.

(v) Favus is a chronic type of ringworm in which dense crusts ( Scutala) develop in the hair follicles which lead to alopecia ( baldness, loss of hair).

(vi) Candida albicans causes candidiasis ( thrush). It can occur in mouth, vagina, intestine, etc.

Heat  and moisture help these fungi to grow in the skin folds such as those in the groin or between the toes. The infection of ringworm is usually acquired from soil or by using towels, clothes or comb of infected persons. 

United StatesEdit

In the United States, infectious diseases is a subspecialty of  internal medicine and pediatrics. In order to "sit" for the infectious diseases' board certification test (administered by the American Board of Internal Medicine, or the American Board of pediatrics), physicians must have completed their residency (in  internal medicine, or pediatrics), then undergo additional fellowship training (for at least 2, or 3 years, respectively). The exam has been given as a subspecialty of internal medicine since 1972 and as a subspecialty of pediatrics since 1994.

 What is communicable illness?

A communicable illness or disease is one that can be transmitted from one person to another through contact with bodily fluids, contaminated objects or surfaces, or through respiratory droplets expelled by infected individuals. Examples of communicable illnesses include the flu, tuberculosis, sexually transmitted infections, and the common cold. The spread of communicable illnesses can be prevented through measures such as vaccination, good hygiene practices, and avoiding contact with infected individuals.

What are 20 common communicable diseases?

Here are 20 common communicable diseases:

1. Influenza (Flu)
2. Tuberculosis (TB)
3. Malaria
5. Hepatitis B
6. Respiratory Syncytial Virus (RSV)
7. Measles
8. Chickenpox (Varicella)
9. Dengue Fever
10. Norovirus Infection (Stomach Flu)
11. Zika Virus Infection
12. Cholera
13. Typhoid Fever
14. Meningitis
15. Pertussis (Whooping Cough)
16. Pneumonia
17. Hepatitis A
18. Shigellosis
19. Gonorrhea
20. Syphilis
21 Coronavirus (2019 Novel Coronavirus) ( COVID _19 )

It is important to note that there are many more communicable diseases, but these are some of the most common ones globally. It's crucial to take preventative measures and seek medical attention when needed to prevent the spread and complications of these diseases.


Popular posts from this blog



Nucleic Acids