Cold sores – also called fever blisters – are a painful infection caused by the herpes simplex virus (HSV). These blisters are usually caused by HSV type 1, but can less commonly be due to HSV type 2. They may show up anywhere on your body, but are most likely to appear on the outside of your mouth and lips, your nose, cheeks, or fingers. Blisters form, then break and ooze; a yellow crust or a scab develops and eventually sloughs off, revealing new skin underneath. The sores usually last 7 to 10 days and are contagious until they crust over completely
Ninety percent of all people get at least one cold sore in their life. This first occurrence is often the worst. Some children who are affected may become seriously ill. After the first infection, many people develop antibodies and never have another cold sore. About 40% of American adults, however, have repeated cold sores
Although cold sores generally are not serious, the infection may be life-threatening for anyone who has AIDS or whose immune system is depressed by other disorders or medications. Patients with severe eczema may also get HSV over large parts of the body.
The infection from a cold sore may cause blindness if it spreads to the eye, and meningitis or encephalitis if it spreads to the brain.