The BBC published an interesting article yesterday on the history of HIV and why certain strains of HIV "succeeded" over others. Perhaps surprisingly, there are four groups of HIV virus (M, N, O, P). While the N, O and P groups of viruses have been shown to small groups of people in Africa (collectively less than 1% of the world's HIV infections), only one has succeeded in infecting humans worldwide. That group is the M group of HIV virus, which presumably infected an individual in Cameroon about 100 years ago.
The success of the M virus has been attributed to its immune evasion strategies in jumping from chimpanzee hosts to human hosts. For example, group M viruses secrete the Vif protein, which can inhibit the function of APOBEC3G, a protein normally implicated in antiviral activity. Group M proteins also secrete a human-specific protein that more efficiently and effectively inhibits the function of tetherin proteins - proteins that block viral release - than the other groups of viruses. These two factors along with globalization, sexual promiscuity, and unsafe sex and use of needles, have created a perfect storm for creating the HIV pandemic that exists today.