There is a relationship between sun exposure and herpes infection. For people with the herpes simplex virus type 1 (the type that can cause cold sores on the lips), flare-ups may develop after sun exposure. Flare-ups can also occur when someone is under emotional stress, ill with fever or menstruating. An injury can also cause a flare-up. Often, flare-ups develop for no apparent reason.

After a person has had chickenpox (due to another herpes virus), the virus can live in the body for many years and "reactivate." This causes a skin infection called shingles. Sun exposure can also trigger shingles.

One reason that flare-ups may develop after sun exposure is that ultraviolet light from the sun can hinder certain immune cells.

In addition to triggering flare-ups, sun exposure may make the sores of a herpes infection more irritated. Many doctors recommend that people avoid direct or excessive sun exposure during herpes outbreaks.

Herpes infections are common. They can be particularly serious in newborns, the elderly and those with suppressed immune function. This is especially true if the infection causes an eye infection, pneumonia or meningitis.

Herpes outbreaks that are triggered or worsen after sun exposure are examples of "photosensitivity." A photosensitive person is prone to problems after sun exposure. Symptoms might include rash (or worsening of an existing rash), fever, fatigue or other problems.