The entry of the causative agent of the disease into the human body and its multiplication there is known as the infection. But every infection does not manifest the disease, as there is a body defense mechanism. This defense mechanism is known as the immunity.
A disease is caused due to a specific organism or by its toxic products. It is transmitted from man, animal, or from environmental agents like food wastes, air, soil and dust, which is called ‘Communicable disease’. Hence, the communicable disease starts from a ‘source’ then it is transmitted to other persons through different ways, namely ‘modes’ of transmission and attacks a susceptible ‘host’ and causes the disease. So, for a disease three factors are required:
(ii) Modes of transmission
(i) Source of Infection:
The source is the starting point of infection, from where the organism or disease agent reaches the susceptible host and cause of the disease. The source may be man, animal, object or substance from which an infectious agent passes to the host.
(ii) Modes of Infection:
Communicable diseases may be transmitted from the source of infection to a susceptible individual in a variety of ways. The mode of transmission of infectious disease may be classified as follows:
(A) Direct Transmission
1. Direct contact:
This means direct and essentially immediate transfer of infectious agents from the source to a susceptible individual, without any intermediate agency for example: skin to skin contact by touching, kissing or sexual intercourse.
2. Droplet infection:
This is- the droplet spray of saliva and secretions from the nose and mouth during sneezing, spitting, talking. The diseases transmitted by droplet spread include common cold, diphtheria, whooping cough, tuberculosis, meningitis etc.
3. Contact with soil:
n case of direct exposure of susceptible tissue to a disease agent in soil, compost or decaying vegetable matter which may result in infection like, hookworm, tetanus etc.
4. Bite of an animal:
The familiar example is that of rabies.
(B) Indirect Transmission
Indirect transmission of disease includes the traditional 5Fs-flies, finger, fomites, food and fluid. Indirect transmission can occur in a variety of settings:
1. Vehicle borne:
Which implies transmission of the infectious agent through the agency of water, food, blood and other biological products such as, tissues and organs? Of these, food and water are the most frequent vehicles of transmission as they are used by everyone.
2. Air borne:
In this case air is the main mode of transmission of disease which occurs through droplet nuclei like tuberculosis, influenza, chickenpox, measles etc. and through dust in case of hospital acquired infection.
3. Fomites borne:
Fomites include soiled clothes, towels, linen, handkerchiefs, cups, spoons, books, toys, lavatory chains, surgical dressings etc. Diseases transmitted by fomites include diphtheria, typhoid fever, bacillary dysentery, hepatitis, eye and skin infections.
4. Unclean hands and fingers:
Hands are the most common medium through which pathogenic agents are transferred to food from the skin, nose, bowel, etc. Diseases transmitted through unclean hands and fingers are typhoid fever, dysentery and intestinal parasites.
(iii) Host (people at risk):
The infectious agent must find a portal entry like respiratory tract, alimentary tract, genitourinary tract, skin etc. On gaining entry into the host, the organisms must reach the appropriate tissue in the body of the host where it may find optimum conditions for its multiplication and survival. Then the disease agent must find a way out of the body (portal exit) in order that it may reach a new host and propagate its species.