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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, which may be bacterial, a parasite, fungal, or a virus, prion or viroid. The scientific study of diseases in medicine caused by biological agents is infectious diseases.

All multicellular organisms are colonized to some degree by extrinsic organisms, and the vast majority of these exist in either a symbioticClownfish (Amphiprion ocellaris) in their Magnificent Sea Anemone (Heteractis magnifica) home. Symbiosis (pl. symbioses) is an interaction between two organisms living together in more or less intimate association or even the merging of two dissimilar org, commensal , or parasitic relationship with the host. An example of the former would be the anaerobicAnaerobic is a technical word which literally means without air''. The presence or absence of air, or more precise the oxygen in the air, affects various chemical and biological reactions. One example is that wood which is totally immersed in water does n bacteria species which colonize the mammalianSubclass Monotremata Monotremata Subclass Marsupialia Didelphimorphia Paucituberculata Microbiotheria Dasyuromorphia Peramelemorphia Notoryctemorphia Diprotodontia Subclass Placentalia Xenarthra Dermoptera Desmostylia Scandentia Primates Rodentia Lagomorp colonIn anatomy of the digestive system, the colon is the part of the intestine from the cecum to the rectum. Its primary purpose is to extract water from feces. In mammals, it consists of the ascending colon on the right side, the transverse colon, the descen, an example of the latter would be the various species of staphylococcusaureus S. capitis S. epidermidis S. haemolyticus S. hominis S. lugdunensis S. saprophyticus S. warneri S. xylosus Staphylococcus is a genus of gram-positive bacteria. Under the microscope they appear round (cocci), and form in grape-like clusters staphyl which exist on human skinThis article is about skin in the biological sense. For other uses of skin see skin (computing) and skinhead. Regarding exposure of the skin to be seen by others, or prevention of this, see nudity, nudism, modesty, clothing. Fruits such as oranges also ha. 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:

• the route of entry of the pathogen and the access to host regions that it gains
• the intrinsic virulenceVirulence is a term used to refer to either ::the relative pathogenicity :or ::the relative ability to do damage to the host of an infectious agent. Virulent bacteriophage Virulent phage infect their bacterial hosts and lyse them after intracellular propa 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.