Cold sores are painful, contagious blisters around the mouth and on the lips. They are caused by herpes simplex virus (HSV) infection.
There are two types of HSV: HSV-1 and HSV-2. HSV infection can manifest in many different ways and is frequently asymptomatic (i.e., without symptoms).[1][2] After the initial infection, HSV goes dormant, but it remains in the body. In some people with HSV, the virus periodically reactivates and causes cold sore outbreaks, which are known as “recurrences,” in roughly the same place as the initial infection.[3] Cold sores are most often due to HSV-1 infection but can also be due to HSV-2.[1][2]