HIV has probably originated from multiple zoonotic transmissions of Simian Immunodeficiency Virus (SIV) from non-human primates to humans in West and Central Africa. There are two HIV types: HIV type 1 (HIV-1) groups M, N, O and P and HIV type 2 (HIV-2) groups A–H. Within the HIV-1 group M, nine subtypes are found, designated by the letters A–D, F–H, J, and K. Within a subtype, changes in the amino acid sequence is observed in the range of 8-17%, but it can be as high as 30%, while differences between subtypes are generally found in the range of 17-35%.
In fact, when new combinations between different HIV-1 subtypes occurs, it results in different Unique Recombinant Forms (URFs), some developed into Circulating Recombinant Forms (CRFs) as propagated in three or more epidemiologically unlinked individuals. The viruses fueling these epidemics vary according to geographical regions, with clade C virus being the most prevalent worldwide, and clade B being currently the most prevalent in the United States and Europe.
Thirty years after the first description of AIDS, an estimated 35.0 million [33.2 million–37.2 million] people were living with HIV at the end of 2013. 2.1 million [1.9–2.4 million] had become newly contaminated with HIV in 2013, including 240000 children, and 1.5 million [1.4–1.7 million] HIV-infected persons died.