Cold sore caused by herpes virus.
A localized viral infection caused by the herpes simplex virus type 1. Fever blisters, also called cold sores, usually involve small areas of skin on the border of the lip, inside the mouth, on the gums or tongue, or on the inner surface of the cheeks. A primary infection is defined as the first time the virus infects the person. This usually occurs in childhood and may not even cause symptoms, although it often causes fever and blisters inside the mouth. Following the primary infection, the virus persists in the nerves near the affected skin in a dormant stage that is intermittently reactivated. Reactivation of the virus produces a fever blister and may be caused by factors including fever, emotional stress, poor nutrition, dental procedures, and sun exposure.
A vesicular rash usually appearing on the lips or mucous membrane of the mouth during another infectious illness. The rash is caused by herpes simplex virus.