Fever blisters, commonly referred to as cold sores, are blisters that occur in or around the mouth. They are caused by the herpes simplex virus. Fever blisters often appear to be raised and filled with a yellowish liquid. Though generally small, they can be quite painful. Some individuals develop a mild fever with active fever blisters.

Fever blisters are extremely common. The herpes virus that most often causes fever blisters is herpes simplex type 1. Within the United States, most individuals are infected with herpes simplex type 1 before they reach 20 years of age.

When a person is initially infected with the herpes simplex virus, he or she may not immediately develop fever blisters. The newly infected individual may develop fever blisters later or not at all. The herpes virus that causes the blisters takes up residence in nerve tissues located in the face. Some individuals develop fever blisters when the herpes virus reactivates. The sores often recur in the same place. However, they don’t cause any serious health issues.

Another form of the herpes virus, herpes simplex type 2, is the virus responsible for genital herpes. This virus also inflicts babies born to herpes-infected mothers at the time of birth. However, herpes simplex type 2 can also cause fever blisters.

Fever blisters, or rather the herpes viruses that cause them, are highly contagious. Both direct and indirect contact with fever blisters can result in infection. As such, kissing and sharing items such as dishes, towels, and personal-care items may lead to infection. Sometimes, fever blisters can be caused by oral-to-genital contact with a person infected with herpes simplex 2. Likewise, genital herpes can be contracted through the same type of contact with a person suffering from active blisters.

The lapse of time from the initial virus infection to the eruption of fever blisters is usually about a week or two. However, it may take up to three weeks for the first flare-up to become obvious. Usually, the blisters last for about a week to ten days. After the initial infection, the herpes virus may remain dormant within the nerve cells, causing a recurrence of blisters on occasion.

After the initial eruption of blisters, new flare-ups tend to be milder. Many individuals develop symptoms before flare-ups, including burning sensations, localized sensitivity, itching, or tingling. These signs may occur up to two days before an outbreak.

Fever blisters usually subside without treatment. However, there are antiviral medications that may help to reduce pain and shorten the duration of symptoms. Some individuals find pain relief by applying cold or warm compresses to the area.