What is a cold sore?

Cold sores are caused by the herpes virus (similar to, but typically not the same virus that causes genital herpes, although you can get genital herpes on your face or mouth herpes on your genitals if you engage in oral sex). They are painful, pus-filled sores that usually erupt around your mouth. Infected individuals typically experience cold sore when their immune system is run down (hence they may appear at the same time that you get a cold) and their body is unable to control the virus.