Herpes labialis is caused by herpes simplex virus type 1 (HSV-1). Infection with type 2 virus can also lead to (primary) herpes labialis, but this type rarely causes a relapse of the infection. The primary infection with HSV-1 usually occurs before the age of 20 years. Antibodies against the virus can be found in about 80% of all adolescents.
After primary infection, the virus hides and assumes a dormant state in the ganglion, where it lies latent throughout the individual’s lifetime. Stimuli such as fever, menstruation, sunlight, and upper respiratory infections can reactivate the virus, after which it returns to the skin epithelial cells through the nerve. In contrast to the primary infection, during which all oral mucosa can be affected, relapsing infections are limited to the mucosa of the hard palate or, in older children and adults, the lips. The number of relapses decreases after the age of 35 years.