Infection

An infection is the detrimental colonization of a host organism by a foreign organism. 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.

Bacterial infections are caused by the presence and growth of microorganisms that damage host tissue. The extent of infection is generally determined by how many organisms are present and the toxins they release. Worldwide, bacterial infections are responsible for more deaths than any other cause.