Fever Blisters and Cold Sores

Basic Facts About Fever Blisters

Young Woman with Cold Sore
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Cold sores or fever blisters are common skin conditions that affect 15 percent to 30 percent of the U.S. population. Cold sores are actually caused by the herpes simplex virus (HSV) and are the most common manifestation of a herpes simplex virus infection.

Fever Blisters and the Herpes Simplex Virus

Fever blisters are caused more often by the herpes simplex virus type 1 (HSV-1) than herpes simplex virus type 2 (HSV-2).

As a matter of fact, more than 85 percent of the world's population has been infected with HSV-1. These infections follow the usual course of herpes simplex virus infections in that the first outbreak is usually more painful and lasts longer than recurrent infections.

Fever Blisters - Primary Infection

The first time the skin in or around the mouth comes in contact with the herpes simplex virus; the outbreak occurs inside the mouth -- on the gums, tongue, and throat. This is called gingivostomatitis. This first infection occurs most often in childhood, and the highest incidence of infection occurs between 6 months and three years of age. Children get pain, fever, swollen lymph nodes, and may have difficulty swallowing. These symptoms last for about a week and resolve spontaneously. Children with gingivostomatitis are at risk for dehydration if the pain keeps them from drinking fluids. Water-based popsicles are sometimes used to provide hydration and pain relief.

Fever Blisters - Recurrent Infection
Once a person has been exposed to the herpes simplex virus, the virus remains in certain cells in the body and can reactivate at any time. Reactivation can be triggered by trauma to the skin, menstruation, sun exposure, stress, fever, and other causes.

This reactivation produces the lesions we know as cold sores or fever blisters. Fever blisters are most often seen on the border of the lip and consist of three to five vesicles. Over the next three to five days, the vesicles become pustular, ulcerative, and then crust over. Symptoms are usually most severe 8 hours after the outbreak. Most people have about 2 outbreaks per year, but 5 percent to 10 percent have greater than 6 outbreaks per year.

Fever Blisters - Prodrome

Recurrent infections are often preceded by a prodrome, symptoms that appear before the outbreak occurs. Common prodromal symptoms for fever blisters are pain, tingling, and burning. A herpes prodrome can last from 2 hours to 2 days.

Spread of Fever Blisters

Fever blisters are contagious and spread by direct contact with infected saliva or droplets in the breath, or by skin to skin contact. The herpes simplex virus can be reactivated in response to various stimuli including UV radiation (sun exposure), stress, a cold, illness, or dental work.


Habif, Thomas. "Warts, Herpes Simplex, and Other Viral Infections." Clinical Dermatology, 4th Edition. Ed. Thomas Habif, MD. New York: Mosby, 2004. 381-388.

Yeung-Yue, Kimberly. "Herpes simplex viruses 1 and 2." Dermatologic Clinics 20(2002): 1-21.

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