For years scientists have searched for the sources of HIV - Human Immunodeficiency Virus. What complicates the effort is the various groups of HIV-1. For example, HIV-1 has four groups: M and N have been traced to chimpanzees. In fact ninety percent, or forty million, of the world's AIDS cases are caused by Group M viruses. But the origins of groups O and P, which have infected over one hundred thousand Africans, were a mystery ' until now.

Researchers have now linked these HIV groups to Gorillas. They did so by screening fecal samples from parts of Africa: western lowland, eastern lowland, and mountain gorillas, for gorilla SIV, the primate version of HIV. Of the four countries where the samples were collected, gorilla SIV was only found in Southern Cameroon.

Not only did the samples resemble HIV-1 group O and P, the genetic information revealed that they originated through a cross-species transmission from chimps to gorillas. These are the same chimps that are the source of groups M and N. SIV has only been found in African monkeys and apes so far.

Some strains cross species and adapt well. This allowed the chimp SIV to infect a gorilla either through a bite or other exposure to bodily fluids. Eventually this became gorilla SIV. In humans, the viruses have adapted so that it spreads efficiently which allowed group M to cause a global pandemic.

Understanding the origins of HIV is crucial if we're to prepare for future risk of additional variations of SIV entering the human population. That risk is a reality so long as humans continue to hunt primates and rely on them as a food source.