Causes of a Cold Sore
Infection by the herpes simplex virus type 1 is the primary cause of fever blisters. When the body is first infected it may not develop a cold sore. However the herpes virus will remain on the nerves in the face. Different circumstances or risk factors will cause the virus to flare resulting in a fever blister. Herpes simplex virus type 2 primarily causes genital ulcers but can, on occasion, cause a cold sore.