We refer to the source of infection as the origin from which a host acquires the infection, either endogenous (i.e. originating from a person's own commensal microbial flora) or exogenous (i.e. an individual, animal or object that in the external environment of the host). Usually the source can be identified as an individual, animal or object in a specific place, and at a specific time.

Thus, a person can be a source of infection; either for him/her self (endogenous) or to other people (directly through personal contact, or indirectly, e.g. by contaminating food or beverages).

In addition to people, also animals can be sources of infection

Objects may be sources of infection; food, water, air-conditioning systems, showers, medical instruments, recreational waters, door knobs, cotton handkerchiefs etc. Most man-made products that may be sources of infection are required to be produced while limiting the risk of contamination. 

In most outbreak investigations, the principal objective is to identify the source of the infection. Interestingly enough this sometimes leads to semantic problems: an identified 'source' (e.g. a chocolate cake) is usually contaminated by some other source (e.g. the baker of the cake, or the eggs used). Tracing back such a 'chain of transmission' usually leads back to the reservoir. In a number of articles the concept of 'source' and 'reservoir' are used as synonyms, though strictly speaking they are not.

Inanimate sources of infection are sometimes referred to as 'vehicle of infection' (e.g. the chocolate cake) or 'fomites' (e.g. the cotton handkerchief). Inanimate sources (vehicles, fomites) are part of the indirect transmission route..

Source of infection should be distinguished from source of contamination (e.g. overflow of a septic tank, contaminating a water supply).

References:

  1. David L. Heymann (editor). Control of Communicable Diseases Manual. APHA, 2008