Human immunodeficiency virus (HIV) is a retrovirus that destroys CD4 T cells and is the aetiological agent of acquired immunodeficiency syndrome (AIDS).  HIV is divided into 2 types, both of which cause AIDS: HIV 1, responsible for the global epidemic; and HIV 2, less pathogenic and restricted mostly to West Africa.  AIDS, which usually occurs after approximately 6 to 9 years of HIV infection, is a constellation of opportunistic and other infections, conditions, or malignancies. These occur as a result of increasing immune depletion over time. 
There are 33.3 million people infected with HIV worldwide, 22.5 million of whom are in sub-Saharan Africa, and the Caribbean is the second most heavily affected region of the world.   AIDS was first identified in the US in 1981, and the US has the most severe HIV epidemic in the developed world. In the US, 49% of all newly diagnosed HIV infections in 2006 were among men who have sex with men.  In contrast, the sub-Saharan epidemic is predominantly heterosexual, with women comprising 60% to 70% of those infected with HIV. Overall, globally, the HIV incidence rate is believed to have peaked in the late 1990s and subsequently stabilised, notwithstanding increasing incidence in a number of countries.