Small (35 to 40 nm), nondeveloped, single-stranded RNA virus. Feline caliciviruses were recognized as cytopathic agents in 1957 and as feline pathogens in 1960. Under experimental conditions, most FCV isolates induce a mild disease characterised by oral ulcers and modest serous nasal or conjunctival discharges. Virulent FCV isolates produce pneumonia, especially if cats are exposed to aerolized virus.