Cold sore on a man's lower lip caused by the Herpes simplex virus (HSV). This common viral infection is characterised by the appearance of painful fluid-filled blisters, commonly called cold sores. It is contagious and usually spreads by direct contact with the blisters or the fluid they contain. Herpes simplex virus exists in two forms known as HSV1 and HSV2. HSV1 is usually associated with infections of the lips, mouth and face, whereas HSV2 affects the genitals. There is no cure for Herpes simplex, and treatment of individual outbreaks depends on their type, site and severity. Antiviral drugs, such as acyclovir, can be used to reduce the severity of an attack.