What causes fever blisters?
A fever blister located on the lips, mouth, and face is usually caused by the highly contagious herpes simplex type 1 virus. Only a small number are caused by herpes simplex type 2, the type usually associated with genital lesions.
There is no cure for a fever blister. However, there are medications that will relieve the pain and itchiness, and speed the healing of the blister.
See: Herpes labialis (oral herpes simplex)