Feline immunoeficiency virus (FIV), occasionally referred to as Feline AIDS is a lentivirus that affects domesticated housecats worldwide. According to Richards (Dec 2005:215-217), 11% of cats worldwide are infected with FIV. According to another study, 2.5% of cats in the USA are infected with FIV (Zislin Dec 2005, p. 219-220). FIV is in the same retrovirus family as Feline leukemia virus (FeLV). FIV differs taxonomically from two other feline retroviruses, feline leukemia virus (FeLV) and feline foamy virus (FFV). Within FIV, five subtypes have been identified based on amino acid sequence differences coding for the viral envelope. FIV is not a death sentence for cats, as they can live relatively healthy as carriers and transmitters of the disease for many years. A vaccine is available although its efficiency remains uncertain, and cats will test positive for FIV after vaccination.