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Fever blisters, also called cold sores, are small sores that usually occur outside the mouth

In ancient Rome, an epidemic of fever blisters prompted Emperor Tiberius to ban kissing in public ceremonies. Today fever blisters still occur in epidemic proportions. About 100 million episodes of recurrent fever blisters occur yearly in the United States alone. An estimated 45 to 80 percent of adults and children in this country have had at least one bout with the blisters.

Fever blisters usually occur outside the mouth--on the lips, chin, cheeks or in the nostrils. When fever blisters do occur inside the mouth, it is usually on the gums or the roof of the mouth. Inside the mouth, fever blisters are smaller than canker sores, heal more quickly, and often begin as a blister.

Currently there is no cure for fever blisters. Some medications can relieve some of the pain and discomfort associated with the sores, however. These include ointments that numb the blisters, antibiotics that control secondary bacterial infections, and ointments that soften the crusts of the sores.

If fever blisters erupt, keep them clean and dry to prevent bacterial infections. Eat a soft, bland diet to avoid irritating the sores and surrounding sensitive areas. Be careful not to touch the sores and spread the virus to new sites, such as the eyes or genitals. To make sure you do not infect others, avoid kissing them or touching the sores and then touching another person.

There is good news for people whose fever blister outbreaks are triggered by sunlight. Scientists at the National Institute of Dental and Craniofacial Research have confirmed that sunscreen on the lips can prevent sun-induced recurrences of herpes. They recommend applying the sunscreen before going outside and reapplying it frequently during sun exposure. The researchers used a sunblock with a protection factor of 15 in their studies.

Little is known about how to prevent recurrences of fever blisters triggered by factors other than sunlight. People whose cold sores appear in response to stress should try to avoid stressful situations. Some investigators have suggested adding lysine to the diet or eliminating foods such as nuts, chocolate, seeds or gelatin. These measures have not, however, been proven effective in controlled studies.

Fever blisters are caused by a contagious virus called herpes simplex. There are two types of herpes simplex virus. Type 1 usually causes oral herpes, or fever blisters. Type 2 usually causes genital herpes. Although both type 1 and type 2 viruses can infect oral tissues, more than 95 percent of recurrent fever blister outbreaks are caused by the type 1 virus.

Herpes simplex virus is highly contagious when fever blisters are present, and the virus frequently is spread by kissing. Children often become infected by contact with parents, siblings or other close relatives who have fever blisters.

A child can spread the virus by rubbing his or her cold sore and then touching other children. About 10 percent of oral herpes infections in adults result from oral-genital sex with a person who has active genital herpes (type 2). These infections, however, usually do not result in repeat bouts of fever blisters.

Most people infected with the type 1 herpes simplex virus became infected before they were 10 years old. The virus usually invades the moist membrane cells of the lips, throat or mouth. In most people, the initial infection causes no symptoms. About 15 percent of patients, however, develop many fluid-filled blisters inside and outside the mouth 3 to 5 days after they are infected with the virus. These may be accompanied by fever, swollen neck glands and general aches. The blisters tend to merge and then collapse. Often a yellowish crust forms over the sores, which usually heal without scarring within 2 weeks.

The herpes virus, however, stays in the body. Once a person is infected with oral herpes, the virus remains in a nerve located near the cheekbone. It may stay permanently inactive in this site, or it may occasionally travel down the nerve to the skin surface, causing a recurrence of fever blisters. Recurring blisters usually erupt at the outside edge of the lip or the edge of the nostril, but can also occur on the chin, cheeks, or inside the mouth.

The symptoms of recurrent fever blister attacks usually are less severe than those experienced by some people after an initial infection. Recurrences appear to be less frequent after age 35. Many people who have recurring fever blisters feel itching, tingling or burning in the lip 1 to 3 days before the blister appears.

Several factors weaken the body's defenses and trigger an outbreak of herpes. These include emotional stress, fever, illness, injury and exposure to sunlight. Many women have recurrences only during menstruation. One study indicates that susceptibility to herpes recurrences is inherited. Research is under way to discover exactly how the triggering factors interact with the immune system and the virus to prompt a recurrence of fever blisters.