A cold sore is a fluid-filled blister that usually appears at the edge of the lips. Other names for a cold sore are fever blister, oral herpes, labial herpes, herpes labialis, and herpes febrilis. Cold sores do not usually form inside the mouth except for the very first time they occur, which distinguishes them from the common canker sores.

Cold sores are caused by a herpes virus. There are eight different kinds of human herpes viruses. Only two of these, herpes simplex, types 1 and 2, can cause cold sores. It is commonly believed that herpes simplex virus type 1 infects above the waist and herpes simplex virus type 2 infects below the waist. This is not completely true. Both herpes virus type 1 and type 2 can cause herpes lesions on the lips or genitals, but recurrent cold sores are almost always type 1.

Oral herpes is very common. More than 60% of...