Two types of HIV have been described: HIV-1 and HIV-2, the latter being less virulent, less transmissible, and confined mostly to West Africa. HIV-1 is phylogenetically close to SIVcpz commensural virus in chimpanzees, which probably arose as a specific transmission event from chimps to humans. However, SIVmac is the etiological  agent of simian AIDS and has many of the features of HIV AIDS. HIV-1 is further divided into three major  groups: M(major), O(outliner), and N(non M or O). Most of the strains responsible for the AIDS epidemic belong to the M group, and this group has been divided into ten different subgroups known as clades (A-K).