PIP: More sophisticated knowledge of the different strains of seemingly closely related retroviruses is essential to understanding acquired immunodeficiency syndrome (AIDS) and developing a successful vaccine. Distinct exogenous human retroviruses include human immunodeficiency virus type 1 (HIV-1), the etiologic agent in AIDS; human T-lymphotropic virus type 1 (HTLV-1), which has been linked with adult T-cell leukemia/lymphoma; and HTLV-II, a virus closely linked to HTLV-I but not definitively implicated in human disease. Closely related to HIV-1 is simian T-lymphotropic virus type 3 (STLV-3 mac). 30-70% of over 1000 African green monkeys tested possess antibodies to STLV-3 proteins. Of interest is the contrasting pathogenicity of HTLV-3 in seropositive Asian macques and African green monkeys; the former always have clinical or pathological evidence of immunosuppressive disease, while the latter remain healthy. Given the apparent ability of African green monkeys to mount an effective immune response against infection with STLV-3, this retrovirus offers a model system for the study of AIDS pathogenesis. It was hypothesized that STLV-3 is a distant progenitor of the AIDS virus, and that HIV-1 may have originated in Africa through the chance transmission of a simian virus to humans. A 4th human T-lymphotropic retrovirus, isolated from healthy Senegalese prostitutes in 1985 and termed HTLV-4/HIV-2, has viral proteins similar to those of STLV-3 and HIV-1. Although HTLV-4/HIV-2 is prevalent among high risk groups in West Africa, it is not associated with immunosuppression; this suggests a unique species-specific pathogenicity similar to that in STLV-3.