WEDNESDAY, July 16 (HealthDay News) -- New research suggests that people of African descent are much more likely to have a genetic trait that makes them more susceptible to infection with the HIV virus. Scientists estimate that the trait -- which also provides protection against a form of malaria -- might account for 11 percent of the HIV cases in Africa, the continent hardest hit by the AIDS epidemic. Overall, the finding shows how the past history of evolution and disease still affects people today, said study co-author Matthew J. Dolan, of the Wilford Hall United States Air Force Medical Center and San Antonio Military Medical Center. "The benefit that the Africans got from a mutation that gave them some resistance to malaria has, statistically at least, rendered them some increased susceptibility to HIV," he said. Researchers have spent years trying to understand why some people who are exposed to the AIDS virus don't get infected. An estimated 70 percent to 90 percent of children born to infected mothers don't develop the disease, and some gay men have avoided it despite repeated exposure. In the new study, a team of researchers studied more than 1,200 members of the U.S. military who became infected with HIV. They wanted to discover more about how genetics affects the disease. The findings were published in the July 17 issue of Cell Host & Microbe. The researchers found that a genetic trait -- found in 60 percent of African-Americans and 90 percent of Africans -- makes HIV infection 40 percent more likely. The trait is virtually nonexistent in whites. The trait also protects people against a form of malaria that is now uncommon. It appears that the genetic makeup of some Africans evolved to give them protection against the form of malaria, Dolan said. Unfortunately, the trait ultimately "set up the African continent for increased susceptibility" to HIV, he said. Dolan estimated that the increased susceptibility could account for millions of extra cases of HIV. On the other hand, people who have the trait live an average of two years longer with the disease once they get it, the researchers found. "It's a two-edged sword," said study co-author Dr. Sunil K. Ahuja, a professor at the University of Texas Health Science Center in San Antonio. Rowena Johnston, vice president of research with the Foundation for AIDS Research, said the new finding provides even more evidence of an evolutionary struggle between humans and disease. But it won't be easy to make the information useful. "Since any one individual has tens of thousands of genes, each of which may influence susceptibility in one direction or another, it's difficult to predict the outcome for any individual with any one particular genotype," she said. Even if Africans or African-Americans discover they have this particular genetic trait, "What would they do with the information?" she asked. As for using the new finding to develop a new anti-AIDS drug, that may be difficult because of the limited effect of the genetic trait, she said.