The herpes simplex virus can remain hidden (dormant) and then become reactivated to cause fever blisters on the lips. This is similar to the varicella virus in chickenpox which can live on in the body for years before becoming reactivated to cause shingles. The onset of fever blisters may be started by other viral illnesses with fever or excessive exposure to sunlight.

Fever blisters present as a few very painful vesicles on the lips which soon form ulcers and then scabs. They can be treated with acyclovir cream if the treatment is started as soon as the discomfort begins. Herpes virus is present in fever blisters and can be spread to others by direct contact such as kissing. Therefore, adults with fever blisters should never kiss a child.

Unlike fever blisters, which occur on the lip, painful aphthous ulcers occur repeatedly on the mucosa of the mouth. The cause of aphthous ulcers is still unknown.