Mathematical model offers clues to Africa epidemic.

It's been known for some time that certain people of European descent have a mutation on their CCR5 coreceptor molecule, and people who have two copies of this mutation appear to be completely protected against HIV infection. However, people of African or Asian descent have neither of these mutations, which has led researchers to theorize that the mutation is the result of some long-ago viral epidemic that never made its way to Asia or Africa.