Infection caused by herpes simplex virus.
Type I typically produces a cluster of small blisters (cold sores, or fever blisters), usually on the lips or face; it can also infect the eyes. Type II, transmitted mostly through sexual contact, causes genital blisters, which rupture, becoming very painful. Oral sex can give either type the chance to infect the usual site of the other. In both types, the virus remains after symptoms end and can reactivate, causing blisters to reappear. Babies born to mothers with active herpes can become infected during birth; this can be prevented by cesarean section. There is no cure, but drugs can reduce severity and risk of transmission.