herpes simplex (hûr´pēz), an acute viral infection of the skin characterized by one or more painful, itching blisters filled with clear fluid. It is caused by either of two herpes simplex viruses: Type 1, herpes labialis, which generally involves the lip (producing what are commonly known as cold sores) or the mouth area (producing canker sores), but can involve the genital area or, in the case of herpes gladiatorum, parts of the body that have been exposed to the virus through skin-to-skin contact in sports; and Type 2, herpes genitalis, which involves the genitals, but may involve the mouth. It is believed that invasion of Type 1 herpes occurs in most persons during infancy and childhood, either as a systemic or severe local infection. Type 2 herpes, or genital herpes, is a sexually transmitted disease that became epidemic in the United States in the late 1960s. Newborns exposed to active herpes in the mother's birth canal can contract a serious form of the disease. The herpes simplex virus can be spread by an infected but asymptomatic person.

Outbreaks of both types of herpes simplex alternate with periods when the virus lies dormant in the nerve cells. The reappearance of blisters may be triggered by such factors as fever, infectious diseases, exposure to sunlight, menstruation, or pregnancy. The blisters usually last from 10 to 14 days. Treatment for recurrent herpes includes elimination of the precipitating conditions, local antibiotic treatment to prevent bacterial infection, and treatment with antiviral drugs such as acyclovir, although some resistant strains (see drug resistance) have developed. There is no cure. The herpes simplex virus is also the cause of a form of viral encephalitis.

See also herpes zoster (shingles).