Interactions of Pathogens with the Host

Infection occurs when a microbe enters a normally nonsterile tissue and may cause an infectious disease with signs and symptoms or a subclinical infection. Infections that are not cleared by the host are characterized as persistent. The agent in nonreplicating or latent infection may begin to grow again or to reactivate. Growth of a microorganism such as normal bacterial flora on a normally nonsterile body surface such as the colonic mucosa in the absence of disease is called colonization. Some infectious agents known as pathogens routinely cause disease; others known as opportunistic cause disease only in persons with altered host defenses, such as acquired immunodeficiency syndrome. The location where a pathogen normally resides is the reservoir, such as naturally infected animals. The mechanism by which the pathogen moves from the reservoir into the patient is known as transmission such as by drinking contaminated water. The portals of entry are mucosal, respiratory, gastrointestinal, genitourinary, and cutaneous. A pathogen may replicate at the portal of entry and cause disease by secreting a toxin or enter the body and spread via the lymphatic or blood vessels, nerves, urinary tract, respiratory tract, cerebrospinal fluid, over mesothelial surfaces, or to the fetus transplacentally. Many infectious agents target a particular organ where they replicate and cause damage, for example, hepatitis, myocarditis, and meningoencephalitis.