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What are cold sores and what causes them?Cold sores (a cold sore), also known as fever blisters, are small sores, or blister-like lesions on the face or inside the mouth. They usually cause pain, a burning sensation, or itching before they burst and crust over. Most commonly, cold sores appear on the lips, chin, cheeks, inside the nostrils, and less frequently on the gums or the palate (roof of the mouth).

The sores are caused by the herpes simplex viruses; the most common cause of sores around the mouth is herpes simplex type 1, or HSV-1. Much less commonly, cold sores may be caused by HSV-2 (herpes simplex type 2), which may result from having oral sex with a person who has genital herpes.