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The following points highlight the two main modes of transmission of communicable diseases in humans. The modes are: 1. Direct Transmission of Communicable Diseases, 2. Indirect Transmission of Communicable Diseases.
Mode # 1. Direct Transmission of Communicable Diseases:
In this, the pathogens are transmitted from an infected person to a healthy person directly without an intermediate agent.
It occurs in following ways:
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(a) By direct contact with an infected person i.e., hand shake, mouth-to-mouth kissing, sexual act, etc. e.g., contagious diseases like chickenpox, smallpox, measles, leprosy, ringworm, gonorrhea syphilis etc.
(b) Droplet infection (though coughing, sneezing and spitting of infected persons) e.g., pneumonia, diphtheria, influenza, tuberculosis (Fig. 8.2), common cold, whooping cough, etc. These are more common in more crowded living conditions.
(c) Contact with soil, e.g., bacterial cysts of tetanus.
(d) Animal bites e.g., rabies viruses.
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(e) Tran placental transmission.
The viruses of German measles and AIDS; and bacteria of syphilis can be transmitted from the maternal blood into foetal blood through placenta.
Mode # 2. Indirect Transmission of Communicable Diseases:
When the pathogens can be transmitted from the reservoir of infection to a healthy person through some intermediate agents.
It occurs in following ways:
(a) Vector borne diseases e.g., malaria (female Anopheles Fig. 8.3), sleeping sickness (vector is tse fly), dengue (Aedes mosquito), cholera (housefly), etc.
(b) Vehicle borne. Pathogens of cholera, dysentery, typhoid etc. are transmitted by agencies like contaminated food, water etc. Such diseases are more common in areas without safe supplies of drinking water. AIDS is spread by blood or semen of suffering donors.
(c) Air borne e.g., influenza, epidemic typhus.
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(d) Formite borne – In this, the pathogens are spread through contaminated articles like handkerchiefs, towels, crockery, etc.
(e) Unclean hands e.g., Ascariasis (Ascaris lumbricoides) and Enterobiasis (Enterobius vermicularis).
Sexually-transmitted diseases (STDs) are those diseases which spread by sexual contact from an infected person to another person e.g., bacterial diseases like syphilis and gonorrhea and viral diseases like AIDS.
