Humans exist in an environment replete with microorganisms, yet only a few of these microorganisms become residents on the skin surface. These resident flora and the skin constitute a complex ecosystem in which organisms adapt to changes in the microenvironment and to coactions among microorganisms. The skin possesses an assortment of protective mechanisms to limit colonization, and the survival of organisms on the surface lies in part in the ability of the organisms to resist these mechanisms. Microbial colonization on the skin adds to the skin's defense against potentially pathogenic organisms. Although microbes normally live in synergy with their hosts, at times colonization can lead to clinical infection. Common infections consist of superficial infections of the stratum corneum or appendages, which can respond dramatically to therapy but commonly relapse. In rare circumstances these infections can be severe, particularly in immunocompromised patients or hospitalized patients with indwelling foreign devices.