Use of an aminopurine derivative to treat latent infection of the herpes virus

A purine derivative of formula (A) or a bioprecursor or a pharmaceutically acceptable salt, phosphate ester and/or acyl derivative thereof is in the preparation of a medicament for the reduction or prevention of herpes virus infection becoming latent and subsequently reactivating in mammals wherein the administration of the medicament commences more than 18 hours post-infection. The preferred purine derivative is famciclovir and the infection is herpes simplex type 1 or 2.