About 10% of the Caucasian population is homozygous for a mutation in either the CCR5 or CXCR4 co-receptor that is required for HIV to enter a CD4 Th cell. In fact, there is a class of drugs that's designed to bind the CCR5 receptors and prevent HIV from ever integrating into the host genome. However, you have to take that drug before you're exposed to HIV at all. It's really probably best utilized in places with endemic HIV like sub-Saharan Africa. The real mystery are the long-term non-progressors. There are certain people who have had HIV for 10+ years yet show no clinical signs of progression to AIDS. That is to say, their CD4 counts are high, their viral load is low, and they don't display any of the opportunistic infections that are associated with clinical AIDS (disseminated tuberculosis, pneumocystis carinii, candidiasis, etc). And I'm not just taking about people who are on the proper anti-retroviral therapy. Long-term non-progressors do better than "regular" (for lack of a better term) people, anti-retroviral therapy not withstanding. It's actually an area of active research. Interesting stuff!