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Understanding Cold Sores -- the Basics

What Are Cold Sores?

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.

What Causes Cold Sores?

Herpes Simplex

Cold sores are usually caused by the herpes simplex virus (HSV) type 1, which is transmitted by such forms of contact as kissing an infected person or sharing eating utensils, towels, or razors. Genital herpes is usually caused by herpes simplex virus type 2. A person with a cold sore who performs oral sex on another person can give that person genital lesions with HSV-1. Similarly, a person who performs oral sex on another person with genital HSV-2 lesions can develop oral lesions.

Sores may develop as late as 20 days after exposure to the virus. Once the virus enters your body, it may emerge near the original site of entry. About two days before an attack you may experience itching or sensitivity at the site. The virus may be triggered by certain foods, stress, fever, colds, allergies, sunburn, and menstruation.

WebMD Medical Reference Reviewed by Debra Jaliman, MD on March 13, 2015

Sources

SOURCES: 

Columbia University College of Dental Medicine: "Cold Sores and Fever Blisters."

Mayo Clinic: "Cold Sore."

Academy of General Dentistry: "What Are Cold Sores?"

MedlinePlus: "Herpes - oral."

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