Herpes simplex
The herpes simplex virus (HSV) is a virus. It causes two common diseases. Both diseases have painful, watery blisters in the skin or mucous membranes (such as the mouth or lips) or on the genitals.
The disease is contagious, particularly when the disease is active. Contagious means that it can spread from one person to another. It is not possible to stop this disease completely after it has started in a person. However, there are some medical treatments that stop the disease for a little while.
An infection on the lips is commonly known as a cold sore or fever blister. A cold sore is not the same as a canker sore, which appears inside the mouth and is caused by something else.