Sunlight is an important causative factor of recurrent herpes simplex.

To evaluate the role of exposure to solar UV radiation (UVR) in primary and recurrent herpes simplex virus 1 (HSV-1) infections, we investigated the self-reported cause of infection among diagnosed patients in Hyogo Prefecture, Japan. Among 4295 infected patients, 3678 had HSV-1, and 2656 of those patients (72.2%) had a recurrent flare-up. Fatigue was the most commonly reported cause of a flare-up among all patients, followed by the common cold and sun exposure. Sun-induced HSV-1 flare-up was reported by 10.4% of the total study population. However, this increased to 19.7% among patients diagnosed in July and August, to 28% among patients younger than 30 years diagnosed in July and August, and to 40% among patients younger than 30 years diagnosed in July and August with a recurrent infection. These results suggest the important role of solar UVR in the development of recurrent HSV-1, possibly due to UVR-induced immunosuppression or direct reactivation of HSV-1 in the neural ganglia.