Feline immunodeficiency virus (FIV) is classified as a member of the genus Lentivirus (subfamily Lentivirinae) of the Retroviridae family on basis of its morphology, biochemical characteristics, genomic organization, Mg'+ dependent reverse transcriptase, and nucleotide sequence homology with other members of this genus. Lentiviruses cause chronic, lifelong infections in their respective host species, which may be followed by a slowly progressive and degenerative disease. FIV was first isolated from a domestic cat suffering from an immunodeficiency syndrome reminiscent of AIDS in humans. Subsequently it was shown that upon FIV infection, domestic cats may develop an immunodeficiency syndrome, hallmarked by secondary and opportunistic infections. The clinical stages preceding the final stage of feline AIDS, are similar to those observed in humans with HIV infection: the acute stage is followed by the asymptomatic carrier (AC) stage, after which a persistent generalized lymphadenopathy (PGL) gradually leads to the stage of AIDS related complex (ARC). Then the animal develops full blown AIDS. The different stages of F1V infection of cats may often not be quite distinct and rapid transitions between the respective stages may be observed (see below). FIV infections occur virtually worldwide in domestic cats and to date three viral clades (A, B, and C) with partially overlapping geographical distributions have been identified on basis of env sequences.[-]