12th Class Biology Microbes in Human Welfare Communicable Diseases

Communicable Diseases.

       (i) 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 –

         (a) The suspected pathogen should be invariably present in the animals suffering from the disease and should not be found in healthy individuals.

         (b) The pathogens isolated from the diseased animal should be grown in a pure culture.

         (c) When this culture is inoculated into a healthy host, the latter should develop the disease and show its characteristic symptoms.

         (d) 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.

         (ii) 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 :

         (a) Viral diseases

         (b) Bacterial diseases

         (c) Protozoan diseases

         (d) Fungal diseases

         (e) Helminthes diseases

         (f) Sexually transmitted diseases (STD)

         (g) Diseases through blood transfusion

         (iii) 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 –

         (a) To know the nature of the disease, i.e., the causative agent and its life-history.

         (b) To find out mode of transmission of the disease, i.e., how the causative agents enter the human body.

         (c) To devise protective measures against the attack of the causative agent of the disease.

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.

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 :

(i) 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.

(ii) 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.

(iii) 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.

(iv) Animal Bites : Virus of rabies, or hydrophobia, is introduced through the wound caused by the bites of rabid animals, most commonly dogs.

(v) 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 :

(i) Arthropod Vectors : Insects transmit diseases in two different ways.

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.

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.

(ii) 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.

(iii) 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.

(iv) 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.

(v) Unclean Hands : The unclean hands may carry disease germs to food or mouth. Therefore, hands should be washed before taking meals.

(vi) 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.

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 :

(i) Exotoxins : These are released as soon as produced. The diseases brought about by exotoxins include tetanus, scarlet fever, diphtheria, and botulism (food poisoning)

(ii) 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.