Lips affected by a viral infection caused by the Herpes simplex virus and known as a cold sore. This is a common and troublesome infection characterised by the appearance of small fluid- filled blisters. It is contagious and usually spread 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 with infections of the genitals. Herpes simplex is treated with antiviral drugs such as acyclovir and antibiotic drugs are also used if the sores become infected with bacteria.