Kaposi varicelliform eruption due to herpes simplex virus (eczema herpeticum) is a serious disease in infants, with occasional fatal outcome. Photoinactivation therapy consisting of local application of methylene blue followed by light exposure resulted in rapid improvement both clinically and virologically. Methylene blue, at a concentration of 105M was found to be viricidal to herpesvirus in the light but not in the dark.
Chang T, Weinstein L. Eczema HerpeticumTreatment With Methylene Blue and Light. Arch Dermatol. 1975;111(9):1174-1175. doi:10.1001/archderm.1975.01630210090012