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An infection is the detrimental colonization of a host organism by a foreign species. The colonizing organism interferes with the normal functioning and perhaps the survival of the host. The infecting organism is referred to as a pathogen.

All multicellular organisms are colonized to some degree by extrinsic organisms, and the vast majority of these exist in either a symbiotic, commensal, or parasitic relationship with the host. An example of the former would be the anaerobic bacteria species which colonize the mammalian colon, an example of the latter would be the various species of staphylococcus which exist on human skin. Neither of these colonizations would be considered infections.

An infection is, in effect, a war in which the infecting organism seeks to utilize the host resources in order to multiply at the expense of the host.

The difference between an infection and a colonization is often only a matter of circumstance. Organisms which are normally non-pathogenic can become pathogenic under the right conditions, and even the most virulent organism requires certain circumstances to cause a compromising infection.

The variables involved in the outcome of a host becoming inoculated by a pathogen and the ultimate outcome include:

As an example, the staphylococcus species present on skin remain harmless on the skin, but when present in a normally sterile space, such as in the capsule of a joint, or the peritoneum, will multiply without resistance and create a huge burden on the host.

A list of infectious diseases of humans is available.