One of the obstacles to treatment of the human immunodeficiency virus (HIV) is its high genetic variability. HIV can be divided into two major types, HIV type 1 (HIV-1) and HIV type 2 (HIV-2). HIV-1 is related to viruses found in chimpanzees and gorillas living in western Africa, while HIV-2 viruses are related to viruses found in the sooty mangabey, a vulnerable West African primate. HIV-1 viruses may be further divided into groups. The HIV-1 group M viruses predominate and are responsible for the AIDS pandemic. Group M can be further subdivided into<unk>  based on genetic sequence data. Some of the<unk>  are known to be more virulent or are resistant to different medications. Likewise, HIV-2 viruses are thought to be less virulent and transmissible than HIV-1 M group viruses, although HIV-2 is also known to cause AIDS.
Subtypes of HIV
Structure and genome of HIV
Epidemiology of HIV/AIDS
HIV disease progression rates
Social history of viruses
HIV set point
Timeline of influenza
Epidemiology of measles