Cold sores around the mouth of a male patient. Cold sores are painful, fluid-filled blisters caused by infection with the herpes simplex virus. The infection is contagious and spread by direct contact with the blisters or their fluid. There are two types of herpes simplex virus; HSV1, which causes infections of the nose, mouth and face, and HSV2, which infects the genitals. There is no cure for herpes simplex infection, but outbreaks can be treated with the antiviral drug acyclovir.