Cold sores, or fever blisters, are a common symptom of herpes infection. Cold sores that appear on lips, mouth, face, cheeks, are a work of herpes simplex virus type 1 (HSV-1). Genital herpes is caused by herpes simplex virus type 2 (HSV-2). Cold sores often occur in a bout, and then vanish for a long time. This is because the virus is active only for a short period of time, and then lays dormant until triggered.