Although the prevalence of feline retroviruses has decreased significantly during the last 20 years, they still occur worldwide and in some areas they are still of veterinary importance. In Europe and the USA, EIAV infection has almost been eradicated. As BIV does not cause disease, it is not studied widely and little information is available on its prevalence. BLV occurs in many countries and is of considerable economic importance. The small ruminant retroviruses CAEV and VMV occur worldwide and in some areas at high frequency. Lentiviruses collectively called SIV affect both, non-primates and primates. They are naturally present in Africa but not in Asia, North and South America. In 2006, HIV-1 and HIV-2 infection was estimated by the United Nations Programme on HIV/AIDS to have affected 39.5 Mio people. The worldwide prevalence among adults was estimated to be 1%. Most affected by the AIDS epidemic is the population living in sub-Saharan Africa. FeLV, BIV, BLV and VMV are usually transmitted by direct or even indirect contact. FIV is predominantly transmitted by bites and via milk. EIAV and CAEV are transmitted via milk, by fomites and iatrogenically. SIV and HIV are transmitted through sexual contact and by contaminated needles and blood. Cross-species transmissions of FeLV and VMV occur occasionally. The HIV epidemic is the result of the zoonotic transmissions of SIV from chimpanzees.