HIV/AIDS was recognised as a novel illness in the early 1980s. An AIDS case is classified as "early" if the death occurred before 5 June 1981, when the AIDS epidemic was formally recognized by medical professionals in the United States. Virus origins All known subgroups of HIV-1 and HIV-2 are thought to have entered humans as distinct cross-species transmissions of Simian immunodeficiency virus (SIV) from primates. SIV exists as distinct lineages that cluster in different species of primates. HIV-1 is most similar to the SIV found in common chimpanzees (SIVcpz) in southeastern Cameroon, giving rise to the notion that HIV-1 emerged from chimpanzees in this area. SIVcpz itself emerged in chimpanzees as the result of a recombination of two separate lineages of SIV known to infect red-capped mangabey, and Cercopithecus species. Early 20th century HIV-1 group M (responsible for the global pandemic) is estimated to have emerged in humans around 1920 near Kinshasa, then part of the Belgian Congo. This estimation was the result of time-scaled evolutionary models being applied to modern samples and retrieved early samples of HIV-1 (M). It had previously been estimated from the genetic differences between ZR59 and DRC60 samples and later samples that HIV-1 group M had jumped into humans in 1908 ± 10 years. 1950s The earliest known sample of HIV-1 is from Kinshasa, Democratic Republic of the Congo (DRC) (formerly Zaire, formerly the Belgian Congo). The sample, named ZR59, was isolated from tissues collected from "a Bantu male" in 1959 and was found to be most closely related to modern genome sequences of HIV-1 (M) subgroup D. In 2008, partial HIV viral sequences were identified from a specimen of lymph node collected from an adult female in Kinshasa, DRC in 1960. This specimen, named DRC60, was around 88% similar to ZR59, but was found to be most closely related to HIV-1 (M) subgroup A isolates. These two specimens are significant not only because they are the oldest known specimens of HIV-1, but because they show that the virus already had an extensive amount of genetic diversity in 1960. Possible cases of AIDS from this period include: Richard Edwin Graves Jr., a 28-year-old World War II veteran who had been stationed in the Solomon Islands. Graves died on 26 July 1952 in Memphis, Tennessee with pneumocystis pneumonia and CMV, which some authors suggest constitutes a sufficient number of opportunistic infections for a clinical course