Microorganisms that cause infectious disease are defined as pathogens. Organisms that are pathogenic under one set of conditions may, under other conditions, reside within or on the surface of the body without causing disease. When organisms are present but do not cause harm to the host, they are considered commensals. When organisms multiply and cause tissue damage, they are considered pathogens, with the potential for causing or increasing a pathogenic process (Chart 7.1). Some organisms that were formerly considered insignificant contaminants or commensals have taken on roles as causative agents for opportunistic diseases in patients with HIV infection or other immunodeficiency syndromes or diseases associated with a compromised health status. Consequently, virtually any organism recovered in pure culture from a body site must be considered a potential pathogen.