Retroviruses.

Retroviruses have assumed tremendous importance since the discovery of the viruses which cause adult T cell leukaemia' and acquired immune deficiency syndrome (AIDS).2 3 The groundwork which led to the relatively recent isolation of these human viruses was laid over many years in studies of animal retroviruses, particularly those inducing leukaemia in chicken, mice, cats, and cattle. The act of faith in pursuing retroviruses as causes of human disease on the assumption that what occurs in animals is likely to have a counterpart in man has, therefore, been entirely justified. There are three subfamilies of retroviruses: oncoviruses, lentiviruses, and spumaviruses, the latter of which are not pathogenic. The oncoviruses and lentiviruses, however, are of great medical and veterinary importance. The oncoviruses as their name implies cause tumours, particularly leukaemias and other diseases of the haemopoietic and lymphoid systems in man, cattle, cats, mice, and chickens. The lentiviruses include human immunodeficiency virus (HIV), which is the cause of AIDS, maedi-visna virus of sheep, equine infectious anaemia virus, and feline AIDS virus.