Cold sores and fever blisters are caused by the herpes simplex virus type 1 (HSV-1), a virus that passes from person-to-person by direct contact with infected skin or secretions, including saliva. The sores usually appear as clusters of tiny blisters on the lip. HSV-1 can cause similar, but smaller, blisters that rapidly become ulcers inside the mouth on the gums and palate (roof of the mouth).
The first time HSV-1 invades the skin, it causes a primary infection, which usually occurs in childhood or adolescence. It may cause a fever, sore mouth and sore throat. The primary infection often is similar to other viral or bacterial throat infections, and often is not diagnosed as an HSV-1 infection.
After this primary infection subsides, the virus remains dormant (inactive) indefinitely in nerve roots near the affected skin area. In some people, the virus becomes active again from time to time and causes new active sores on the skin, usually on the lip. These sores have become known as cold sores or fever blisters because the virus can be reactivated by a cold or fever. The virus also can be triggered to become active again by other factors, including stress, trauma to the lips, exposure to the sun's ultraviolet rays, menstruation and suppression of the immune system.
Symptoms of a primary HSV-1 infection may last for one to two weeks. After this infection subsides, HSV-1 remains dormant in a patient's nerve cells for the rest of his or her life. Cold sores and fever blisters usually crust within 4 days and heal completely within 8 to 10 days.
A primary herpes infection — the first time a person is infected — causes fever, headache, nausea, vomiting, painful swelling and open sores on the gums and inside the cheeks, and a painful throat infection that is often mistaken for "strep throat," or another form of bacterial infection in the throat. These symptoms usually begin approximately a week after exposure to someone with HSV-1.
You can help to prevent a primary herpes infection in children by not allowing them to be kissed by anyone who has cold sores, fever blisters or signs of a primary herpes infection. However, despite these protective measures, most children will be infected with HSV-1 by the time they reach adulthood. Several different vaccines are being developed against HSV (types 1 and 2), but these appear to only protect people who have never been infected.
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