What Are Cold Sores?

Cold sores, sometimes called fever blisters, are small eruptions on the lips and in the mouth, caused by herpes simplex-1 virus (HSV-1). There may be a single fluid-filled blister, or a patch of several may appear. When the blisters burst, an unsightly crust forms over the top. Cold sores can also be quite painful.

The HSV-1 virus is closely related to herpes simplex-2 (HSV-1), which causes genital herpes. Both types can affect the mouth and genitals, as well as skin elsewhere on the body and in the eyes. You can get herpes simplex-2 from having oral sex with someone who has a cold sore.

About two-thirds of Americans have the HSV-1 virus in their systems. Some people never have a symptom, while others have frequent outbreaks.