Unknown to some people, and perhaps to many people, a cold sore is a sign of herpes. Cold sores are caused by herpes simplex 1, one of two strains of the herpes simplex virus. What is referred to as a cold sore is actually a blister or cluster of blisters. These blisters are an indication that the herpes simplex virus is at the skin's surface. When blisters are visible, herpes simplex is at its most contagious. Contact with active cold sores should be avoided to prevent spreading. It's possible for a herpes cold sore infection to be spread to the genital region of another person, though this doesn't happen frequently. Cold sores do not represent a serious threat to overall health.