Fever blisters that occur on and around the lips are caused by the herpes simplex virus, according to WebMD. There are two types of the herpes simplex virus that can cause sores on the mouth and genitals: HSV-1 and HSV-2.
Normally, the virus enters the body through a small break in the skin on the mouth, as WebMD notes. The virus spreads from one person to another by such activities as kissing an infected person, using the same silverware and sharing personal items, such as razors. Fever blisters normally take up to two weeks to heal. Treatment helps blisters heal only one or two days faster, but treatment is beneficial when they cause pain.