Control of virus infections, like any other kind of infection control, can be effected either as a prophylactic measure or therapeutically, in order to control and alleviate a virus infection which has already been established in the host animal. Virologists have sought chemical means of combating virus infections. Chemotherapy against virus infections would be made more attractive if it were possible to target the chemical selectively to the infected cells, thereby killing them while sparing the neighboring uninfected cells. In connection with virus infections the term photochemotherapy has traditionally referred to the application of light-sensitive dyes, such as neutral red and proflavine, to the treatment of topical herpes simplex infections. This form of therapy, otherwise referred to as photodynamic therapy, was used in many cases to treat cold sores, genital sore, and keratitis, with some success. The dyes are themselves mutagenic, and thus may give rise to undesirable herpes virus mutants, or deleterious mutations in the cells.