"AIDS was diagnosed and recognized as a new disease in 1981 in the USA with the subsequent discovery of HIV-1 as the causative agent in 1983. The two types of HIV identified to date, HIV-1 and HIV-2, display similar morphology, tropism, and modes of transmission; however, they are genetically and antigenically divergent. HIV-1 is responsible for over 95% infections worldwide, and the different strains of HIV-1 are classified into four groups: major (M), non-outlier (N), outlier (O), and P. The group M HIV-1 is the predominant circulating strain responsible for >90% infections worldwide, and hence, for the global HIV/AIDS epidemic; viruses belonging to the other three groups are endemic in certain African countries and cause fewer infections. HIV arose from cross-species zoonotic transmissions of simian immunodeficiency viruses (SIV) from monkeys to great apes and ultimately to humans. Four independent cross-species transmissions of SIV from chimpanzees (SIVcpz) or gorillas (SIVgor) to humans gave rise to the four HIV-1 groups: M and N from SIVcpz, and O and P from SIVgor. Notably, the pandemic HIV-1 group M strain arose in Cameroon almost a century ago from a single transmission event involving an SIVcpz-infected chimpanzee and a human. The group M strains are further classified into nine subtypes (A, B, C, D, F, G, H, J, and K); each subtype is genetically distinct but phylogenetically equidistant from each other. Two or more of these subtypes can further recombine their genetic material to generate mosaic strains known as circulating recombinant forms (CRFs); around 97 CRFs have been reported to date. The globally dominant HIV-1 subtype C accounts for nearly 50% infections worldwide and is concentrated in Southern Africa and India. The HIV-1 subtype B is dominant in the Americas, Western Europe, and Australasia, and accounts for around 10% global infections. Unlike HIV-1, HIV-2 is largely endemic in West Africa, although the virus has spread to other parts of world in the past decade. Approximately 1-2 million of the PLHIV are infected with HIV-2. The different strains of HIV-2 are classified into nine different groups-A to I, which arose from nine independent cross-species transmission events involving the SIV from sooty mangabey monkeys (SIVsmm). Relative to HIV-1 infection, HIV-2 infection is generally marked by lower viral load, longer asymptomatic period, slower target cell depletion and disease progression, and lower transmission rates. However, in the absence of cART, HIV-2 infection eventually leads to AIDS and, ultimately, death."