An infection is the detrimental colonization of a host organism by a foreign species. An infection is, in effect, a war in which the infecting organism seeks to utilize the host's resources in order to multiply at the expense of the host. The infecting organism, or pathogen, interferes with the normal functioning and perhaps the survival of the host. Colloquially, a pathogen is usually considered a microscopic organism though the definition is broader, including bacteria, parasites, fungi, viruses, prions, and viroids. The branch of medicine that focuses on infections and pathogens is Infectious Disease.
All multicellular organisms are colonized to some degree by extrinsic organisms, and the vast majority of these exist in either a symbiotic or commensal 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. 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:
- the route of entry of the pathogen and the access to host regions that it gains
- the intrinsic virulence of the particular organism
- the quantity or load of the initial inoculant
- the immune status of the host being colonized
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.