An infection is the detrimental colonization of a host organism by a foreign species. In an infection, the infecting organism seeks to utilize the host's resources to multiply, usually at the expense of the host. The infecting organism, or pathogen, interferes with the normal functioning of the host and can lead to chronic wounds, gangrene, loss of an infected limb, and even death.

The host's response to infection is inflammation. Colloquially, a pathogen is usually considered a microscopic organism though the definition is broader, including parasites, fungi, viruses, prions, and viroids. A symbiosis between parasite and host, whereby the relationship is beneficial for the former but detrimental to the latter, is characterised as parasitism.

The branch of medicine that focuses on infections and pathogens is infectious disease. "When infection attacks the body, anti-infective drugs can help turn the tide of battle. Four types of anti-infective drugs exist: antibacterial, antiviral, antitubercular, and antifungal.

A secondary infection is an infection that occurs during or following treatment of another already existing primary infection. Wound colonization refers to nonreplicating microorganisms within the wound, while in infected wounds replicating organisms exist and tissue is injured. All multicellular organisms are colonized to some degree by extrinsic organisms, and the vast majority of these exist in either a mutualistic or commensal relationship with the host.

An example of the former would be the anaerobic bacteria species which colonize the mammalian colon, and 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 non-pathogenic can become pathogenic under the right conditions, and even the most virulent organism requires certain circumstances to cause a compromising infection. Some colonizing bacteria, such as Corynebacteria sp. and viridans streptococci, prevent the adhesion and colonization of pathogenic bacteria and thus have a symbiotic relationship with the host, preventing infection and speeding wound healing.

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

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