EVOLUTIONARY BIOLOGY OF HUMAN IMMUNODEFICIENCY VIRUS

The human immunodeficiency viruses. HIV-l and HIV-2. are members of a group of closely related viruses found in a number of different African primate species. More distantly related lentiviruses are found in several different mam­ malian orders. All are associated with long-term infections, but the outcome of infection ranges from a complete absence of symptoms to a rapidly devel­ oping immunodeficiency and death. While HIV-2 is probably directly related to a virus that is responsible for an asymptomatic infection in the Sooty Mangabey, no obvious candidate for the progenitor of HIV-1 has yet been found. Substantial genetic diversity is present in all immunodeficiency viruses, and phylogenetic analysis of HIV -1 sequences obtained from a wide range of geographic locations has revealed 5-7 groups of viral strains, all equally distant from each other. All groups have been found in Africa, but their distribution elsewhere reflects chance links between individuals at high risk of infection. In some areas large epidemics have spread through groups of such individuals to infect thousands within a few months, resulting in an increase in the global frequency of the particular strain