The herpes simplex virus is the cause of fever blisters, according to the National Institute of Dental and Craniofacial Research. There are two types of the herpes simplex virus, and type 1 causes about 95 percent of all fever blisters.
The herpes simplex virus is contagious when fever blisters appear, and it usually spreads by kissing, as the National Institute of Dental and Craniofacial Research explains. Children may develop fever blisters through contact with others in the household. In turn, children give the virus to others by rubbing a fever blister and then touching other people. Most people with the type 1 herpes simplex virus contract it before the age of 10.
Most people do not have symptoms when the initial infection of herpes simplex occurs, claims the National Institute of Dental and Craniofacial Research. However, a small number of people develop blisters around the mouth anywhere from three to five days after the initial infection. People may also develop swollen neck glands, fever or general aching. A yellow crust may form over the sores, which take about two weeks to heal.
Once the herpes simplex virus is in the body, it is always in the body, according to the National Institute of Dental and Craniofacial Research. The virus stays in a nerve near the cheek bone, but it may travel to the skin's surface, which causes more fever blisters to form.