What causes cold sore?

A virus infection: Cold sores on the lips or near the lips, are usually caused by oral herpes virus. They start as a slightly raised reddish area, then form a cluster of tiny blisters, which break open the next day, leaving a sore (ulcer) to heal up over a week. Sores appear more often when a person is under stress or is ill from another illness. (Babies with herpes should see a doctor if a cold sore appears.)