Cold sore: A small sore located on the face or in the mouth that causes pain, burning, or itching before bursting and crusting over. Common locations for cold sores are the lips, chin, cheeks, and nostrils. Cold sores more rarely appear on the gums and the roof of the mouth. Cold sores are caused by herpes simplex type 1 virus, which lies dormant in the body and is reawakened by factors such as stress, sunburn, or fever from a wide range of infectious diseases, including colds. Sunscreen (SPF 15 or higher) on the lips prevents recurrences of herpes due to sunburn. The virus is highly contagious when fever blisters are present. It is spread by physical contact, such as kissing. Also known as labial herpes, febrile herpes, and fever blister.