Herpes simplex (from the Greek ἕρπης /΄erpis/) is a viral disease caused by herpes simplex viruses; both herpes simplex virus 1 (HSV-1) and herpes simplex virus 2 (HSV-2) cause herpes simplex. Infection with the herpes virus is categorized into one of several distinct disorders based on the site of infection. Oral herpes, the visible symptoms of which are colloquially called cold sores, infects the face and mouth. Oral herpes is the most common form of infection. Infection of the genitals, commonly known as herpes, is the second most common form of herpes. Other disorders such as herpetic whitlow, herpes gladiatorum, ocular herpes (keratitis), cerebral herpes infection encephalitis, Mollaret's meningitis, neonatal herpes, and possibly Bell's palsy are all caused by herpes simplex viruses.

Herpes viruses cycle between periods of active disease—presenting as blisters containing infectious virus particles—that last 2–21 days, followed by a remission period, during which the sores disappear. Genital herpes, however, is often asymptomatic, though viral shedding may still occur. After initial infection, the viruses move to sensory nerves, where they reside as life-long, latent viruses. Causes of recurrence are uncertain, though some potential triggers have been identified. Over time episodes of active disease reduce in frequency.

Herpes simplex is most easily transmitted by direct contact with a lesion or the body fluid of an infected individual. Transmission may also occur through skin-to-skin contact during periods of asymptomatic shedding. Barrier protection methods are the most reliable method of preventing transmission of herpes, but they merely reduce rather than eliminate risk. Oral herpes is easily diagnosed if the patient presents with visible sores or ulcers. Early stages of orofacial herpes and genital herpes are harder to diagnose; laboratory testing is usually required. Prevalence of HSV infections varies throughout the world. Poor hygiene, overcrowding, lower socioeconomic status, and birth in an undeveloped country have been identified as risk factors associated with increased HSV-1 childhood infection. Additional studies have identified other risk factors for both types of HSV.

There is currently no cure for herpes; no vaccine is currently available to prevent or eliminate herpes, although vaccines of varying effectiveness are currently in phase III trials. Also, treatments are available to reduce viral reproduction and shedding, prevent the virus from entering the skin, and alleviate the severity of symptomatic episodes.

Herpes simplex should not be confused with herpes zoster, which is a viral disease caused by varicella zoster virus.

Disorders

HSV infection causes several distinct medical disorders. Common infection of the skin or mucosa may affect the face and mouth (orofacial herpes), genitalia (genital herpes), or hands (herpes whitlow). More serious disorders occur when the virus infects and damages the eye (herpes keratitis), or invades the central nervous system, damaging the brain (herpes encephalitis). Patients with immature or suppressed immune systems, such as newborns, transplant recipients, or AIDS patients are prone to severe complications from HSV infections. HSV infection has also been associated with cognitive deficits of bipolar disorder, and Alzheimer's disease, although this is often dependent on the genetics of the infected person.