The human immunodeficiency virus (HIV) is a retrovirus which causes immune suppression mainly through depletion and destruction of CD4 lymphocytes. This results in increased susceptibility to opportunistic infections which in turn leads to acquired immune deficiency syndrome (AIDS). Since HIV (HIV-1) was first identified in 1983, the infection has spread across the world and has developed into arguably the most important and far-reaching pandemic of this century. HIV infection can be acquired through sexual contact, blood or blood products (including contaminated injecting equipment) and vertically from mother to child. Heterosexual acquisition is the dominant route among women, although in some settings injecting drug use (IDU) can also be an important source. In 1985, HIV-2 was isolated in a study of Senegalese women and subsequently found to be prevalent in West Africa and in areas of emigration from West Africa. HIV-2 is less transmissable and pathogenic than HIV-1 and this review is restricted to HIV-1 infection.