It is caused by herpes simplex virus type 1. The virus lies latent in the body and is reawakened 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) on the lips prevents recurrences of herpes from sunburn.
The virus is highly contagious when fever blisters are present. It is spread by kissing.
A patient with fever blisters should be careful not to touch the blisters and spread the virus to new sites, such as the eyes or genitals.