Infection with FHV is the most common cause of eye disease in cats. After an initial infection with sneezing and runny eyes, the virus hides in the nervous system and may not cause disease for years. During times of stress and other infections the virus becomes re-activated and can then cause severe eye disease. Medicated eye drops currently available either have to be administered very frequently or are irritating to the eye, and are therefore not practical. Cidofovir is a new drug active against FHV and may be effective, even if used only once a day. Therefore, in this study it will be determined whether cidofovir is useful for FHV treatment of naturally infected cats.