Herpes simplex is a virus that can cause painful blisters and sores. The virus usually affects the skin and the thin layers of tissue that moisten, lubricate, and protect certain parts of the body (mucous membranes).

The two main types of herpes simplex viruses are type 1 (HSV-1) and type 2 (HSV-2). Both types of the virus can cause sores on the lips (cold sores) and sores on the genitals (genital herpes). However, HSV-1 more often causes cold sores, while HSV-2 more often causes genital sores. The viruses are transmitted by direct contact, such as touching a sore directly or through sexual contact.

A herpes infection may cause only a single outbreak of sores, but in many cases the person will have recurrent outbreaks. Recurring infections can be triggered by factors such as stress, fatigue, sunlight, or another infection, such as a cold or flu.

Treatment can reduce the number and severity of outbreaks but cannot cure the infection.

Herpes zoster, another form of herpes virus, causes chickenpox and shingles.