Cold sores — also called fever blisters — are tiny, fluid-filled lesions that occur on and around your lips. These blisters are often grouped together in patches. After the blisters break, a crust forms over the resulting sore. Cold sores usually heal within two weeks.
Cold sores spread from person to person by close personal contact, such as kissing. Cold sores are caused by a herpes simplex virus (HSV-1) closely related to the one that causes genital herpes (HSV-2). Both of these herpes simplex viruses can affect your mouth or your genitals, and can be spread via oral sex.
There's no cure for HSV infection and the blisters may recur sporadically — often in response to stress or a weakened immune system. Antiviral medications can help cold sores heal more quickly and may reduce the frequency of recurrences.