Cold sores are caused by a virus, called Herpes simplex virus type 1. The virus enters through the skin around the mouth and causes ulcers to form on the skin surface. After the initial infection, the virus becomes inactive and remains in the nerves that provide feeling to the face. Under periods of physical or emotional stress, the virus can become activated again, which results in an episode of cold sores.

About 90 percent of the population is exposed to the herpes virus that is responsible for cold sores.