HTLV-1, or human T-cell lymphotropic virus 1, the first identified human retrovirus. Discovered by Dr. Robert C. Gallo in 1978. Common in parts of Japan, the Caribbean, Africa and the U.S. Can cause leukemia or brain disease in about 1 percent of those infected.

STLV-1, the simian version of HTLV-1, infecting a variety of Asian and African primates.

HTLV-2, Discovered by Gallo in 1982. Causes a type of leukemia in very rare cases. THE AIDS-RELATED VIRUSES

HIV, or HIV-1, the virus that causes human AIDS. Discovered independently in 1983-84 by Dr. Luc Montagnier at the Pasteur Institute in Paris, who called it LAV (lymphadenopathy-associated virus), and by Gallo at NCI, who called it HTLV-3. Devastates the immune system, causing fatal AIDS in a high but still unknown percentage of those infected. Widespread, especially in the U.S., the Caribbean and central Africa.

SIV, or simian immunodeficiency virus. Also known as simian AIDS or the monkey virus. Discovered by Dr. Myron (Max) Essex and Dr. Phyllis J. Kanki, of the Harvard School of Public Health. Infects captive macaque monkeys, in whom it causes disease, and African green monkeys, in whom it does not. Originally called STLV-3.

HIV-2, a so-called "missing link" between the monkey virus and the AIDS virus. Discovered in 1985 by Essex and Kanki, who initially named it HTLV-4. Genetically very close to the monkey virus. Other almost identical forms have been identified by Montagnier, who called it LAV-2, and by Swedish scientists, who called it SBL 6669. Infection rates are highest in West Africa, where its possible role in causing disease is under intensive study. Montagnier has reported AIDS-like symptoms in 30 HIV-2-infected West African patients treated in Europe, Essex's group has found little evidence of disease in HIV-2-infected Senegalese prostitutes followed for more than 2 1/2 years.