An infectious agent has characteristics which are peculiar to itselt as well as some which it has in common with others. Each agent also has certain limitations, particularly those relating to conditions necessary for continued growth and survival. In general, the more common and severe infections of man are caused by infectious agents which multiply and maintain themselves best in or upon the tissues of man; in this respect man is his own greatest enemy. Man is the reservoir, so to speak, of disease agents which are more likely to be dangerous to himself than to other species.

There are, of course, other natural reservoirs of infection which serve as depots for the propagation and maintenance of infectious agents. Sometimes reservoirs are found among both man and animals. In one relationship man generally serves as the source of infection, while animals act as such onty occasionally. In the second instance, the reverse is true: animals may be the sole reservoir, or they may serve as such in conjunction with insects. Insects are known to act as reservoirs. Last, the inanimate environment may serve in this capacity.

1. Man: Under the following circumstances man is the source or reservoir of infection.

(a) Typical casc: A disease is recognized, the sick person is the source of infection, and others may become ill because of exposure to him.

(b) A typical or missed case: Whether willfully or not, evident disease is not recognized or reported, resulting in danger of infection to persons who come in contact with the case.

(c) The carrier: Carriers are of three types - healthy, incubatory, and convalescent. Persons may be healthy carriers, harboring pathogenic microorganisms without infection resulting. A carrier state without infection is doubted by some authorities. Persons invaded by a microorganism usually are infectious during the latter part of the incubation period and immediately prior to the occurrence of symptoms. Persons who incubate a disease and carry organisms in this manner are called incubatory carriers. Last, patients may be infectious to others during the convalescent period; these are called convalescent carriers.

2. Animals: Animals are reservoirs of the infectious agents which cause rabies, glanders. and anthrax. Ingestion of a contaminated food product like milk may result in typhoid fever, undulant fever, or tubercutosis. Animals may serve as the reservoir of an infection common to them which is transmitted from animal to animal by means of insects but which, under certain conditions, is conveyed to man. An example of this is typhus fever. Under ordinary or endemic conditions, this disease affects rats and is transmitted to man by the rat flea or rat louse. Under epidemic conditions, however, man becomes infected by the rat flea, and spread of the disease from man to man occurs through the bite of the human body louse. Similar conditions prevail in respect to plague.

3. Insects: In a few diseases an insect or an insectlike organism is the reservoir, and infection is maintained in the insect.

Rocky Mountain spotted fever is an example. A wood tick is responsible for harboring the organism, and the female tick may pass on the infectious agent in her eggs. In fact, offspring may be infected through the agency of the male tick. Human beings are infected accidentally by their presence in areas where infected wood ticks are prevalent.

4. Inanimate environment: An inanimate source is the street dust, garden soil, and manure pile, which may harbor tetanus bacilli. Dust storms have carried dust hundreds of miles, with organisms surviving to grow on culture media. In hospitals where patients with infectious diseases are cared for, dust and fluff from bedclothes and linen as well as from walls and floors contain pathogenic bacteria which may be infectious to others. Diphtheria, scarlet fever, and certain virus infections may be contracted in this way.

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