A source of infection is something from which an infectious agent (pathogen) passes on to the new host. This 'something ' can be a person, animal, object or substance. A reservoir is something in which a pathogen lives and multiplies, and on which it depends for survival. This reservoir can also be a person, animal, plant, soil, a substance, or a combination of these. A reservoir is the natural habitat for the pathogen to live and multiply. A reservoir however is not always the source of infection. Source is generally supposed to be the immediate source of infection and may or may not be a part of the reservoir. For example, in hookworm infection the reservoir is man, but the source of infection is the soil contaminated with the infective larvae, whereas for tetanus the source and reservoir is the same, that is the soil. In typhoid fever, the reservoir of infection may be a patient or a carrier, but the source of infection may be the faeces or urine of patients, or contaminated food, water or milk.
There are three kinds of reservoirs: human, animal and non-living things. We will discuss each of these three in a little detail.
Human Reservoir
This is the most common reservoir of infection for humans. The diseases for which man is a reservoir are many - influenza, measles, chickenpox, mumps, worms, AIDS, polio. There are two kinds of human reservoirs :
a) Persons who have the disease: The presence of the disease in the person may be of two different orders -clinical, which means the person has the disease with full-blown signs and symptoms which can be clearly identified, and sub- clinical which means that the person has the disease agent in him but the disease does not manifest itself by signs and symptoms.
While the clinical cases are very obvious and ought to be kept at an arms distance as a dangerous reservoir and source of infection, it is the sub-clinical cases which create the most problems. They do not know that they are carrying the pathogen, and in moving around and mixing with people, they spread the infection. They cause much more damage than the clinical cases of the disease do. These sub-clinical cases can be detected only by laboratory tests.
With the exception of a few infections diseases like measles, sub-clinical infections occur in the case of most infectious diseases. In the case of some diseases like mumps, polio, hepatitis A and B, influenza and diphtheria, a great deal of sub-clinical infections keep occurring. In fact, it is the sub-clinical infections occurring frequently during a person:s life time that help him develop immunity to a variety of diseases. Immunity will be discussed later in this unit.
A third order of infection in a person is what is called latent'infection. This is 'different from sub-clinical infection. In latent infections, the person after having acquired an infection is not able to get rid of it completely. Even after supposedly being cured of the infection, the infectious agent remains in the person, but in a dormant state, without any symptoms. Very often even its presence in blood, tissues and bodily secretions cannot be detected. Herpes simplex is an example of a latent infection. It is there in the body lying dormant until suitable conditions prevail for it to erupt.
b) Persons who have the infectious agent in their system, but do not show any evident disease : They are immune to the disease, hut are a source of infection to others and as such are termed as carriers of specific disease or infection. As a rule, carriers are less infectious than cases we have described under (a) just before this, hut they are more dangerous because they do not get detected and, living a normal life among other people, continue to infect susceptible individuals.
Animal Reservoir :
Birds and animals arc also sources of infection. The infections and diseases which can he transferred to man from birds and animals are called zoonoses. There are over 100 zoonotic diseases that man can catch from birds and animals. Some examples are playable, rabies, yellow fever, Q-fever and Kala-azar. Wild birds, in particular, play an important role in spreading the mosquito-borne encephalitis and several other mosquito-borne febrile diseases. On migrating from one locality to another, they may carry ticks infected with viruses and rickettsiae that may cause disease in humans.
Non-living Reservoir :
Soil and other inanimate matter can also act as reservoirs of infections. Soil may harbour agents that cause tetanus, anthrax and mycetoma, and these agents can get into humans or other animals.
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