A small sore situated on the face or in the mouth that causes pain, burning, or itching before bursting and crusting over. The favorite locations are on the lips, chin or cheeks and in the nostrils. Less frequented sites are the gums or roof of the mouth (the palate).

Fever blisters are caused by herpes simplex virus type 1. It lies latent (dormant) in the body and is reawakened (reactivated) by factors such as stress, sunburn, or fever from a wide range of infectious diseases including colds. Recurrences are less common after age 35. Sunscreen (SPF 15 or more) on the lips prevents recurrences of herpes from sunburn.

The virus is highly contagious when fever blisters are present. It is spread by kissing. Children become infected by contact with someone who has a fever blister and then they spread the virus by rubbing their cold sore and touching other children. A person with fever blisters should be careful not to touch the blisters and spread the virus to new sites, such as the eyes or genitals.