I noted on the Main Page that the frequency of positive HIV tests varies in a regular way with age, sex and race. Be it blood donors, or women who have just given birth, or disadvantaged youths in the Job Corps, or Marines or sailors or soldiers or potential recruits--no matter what group of people happen to be tested, they always test positive according to their racial ancestry,
in the following sequence, and usually in roughly the same ratios:

Asian  <  White  <  Native American  <  Hispanic  <  Black
in the ratios
     ~0.65   <    1.00    <               ~1.6            <      ~ 2.7    <     ~ 5        

In other words, Asians are usually “HIV-infected” only 2/3 as often as Whites, whereas Blacks are usually “infected” about 5 times more often than Whites.

As I noted under What is HIV?, these and  other regularities show that positive HIV tests have something to do with physiology. Why would that vary by race?

Races are distinguished by such things as color of skin, texture of hair, and facial structure. Those are obvious signs of certain genes, or patterns of genes. The genes associated with the immune system are also known to have different patterns in the different races; that is why certain medical conditions are found more frequently among some racial groups than among others.
Since "HIV" seems to measure a response to some sort of health challenge,  the observed racial dependence makes perfectly good sense: race-correlated genes influence the degree of reaction to certain physiologic stresses.

Moreover, there is a ready explanation, why Black people might show a stronger response than others:
The dark skins of Africans have a known physiologic function. Sunlight absorbed through the skin catalyzes the formation of vitamin D, which is both essential but also harmful at too large doses. In equatorial regions, unfiltered sunshine would produce too much vitamin D. Humans evolved in Africa with dark skin for the optimum degree of sunshine-filtering. As humans migrated out of Africa into northern and temperate regions, more of the incident sunshine was needed in order to manufacture sufficient vitamin D, and so skin tones became lighter.
As well as much sunshine, tropical regions also harbor a great variety of bacterial, microbial, parasitic, and viral diseases. It would therefore be curious if humans, evolving in Africa, had not acquired strong immune responses against a wide range of those challenges to health. As humans migrated to other, non-tropical parts of the world where challenges to the immune system were less frequent, it seems reasonable that the responses generated by the immune system might have become somewhat weakened. So we might expect stronger immune systems among people of relatively recent African ancestry than among people whose ancestors migrated out of Africa about 200,000 years ago, whose immune systems had weakened just as the hues of their skins had lightened.
This page was last updated: 11 September, 2007
Why does HIV discriminate by race?
That article also shows that the peculiar geographic distribution of "HIV" in the U.S. can be simulated quite well with just two parameters:
(1) greater health challenges where the population density is greatest, and
(2) immune responses correlated with racial ancestry
These and related points are detailed and documented in
Demographic characteristics of HIV: III. Why does HIV discriminate by race?         
Back to Main Page
These web pages and the mentioned articles
have now been superseded by this book:
The Origin, Persistence and Failings of HIV/AIDS Theory
McFarland,  April 2007, ISBN 978-0-7864-3048-2