The herpes simplex virus (HSV) (also known as Cold Sore, Night Fever, or Fever Blister) is a virus. It causes two common viral infections. Each infection has painful, watery blisters in the skin or mucous membranes (such as the mouth or lips) or on the genitals. The disease is contagious, particularly during an outbreak. Contagious means that it can spread from one person to another.It is incurable with present medicine. An infection on the lips is commonly known as a "cold sore" or "fever blister," though this should not be confused with a canker sore, which appears inside the mouth and is not caused by the herpes simplex virus.