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.