HIV is a huge problem in Africa
A gene which apparently evolved to protect people from malaria increases their vulnerability to HIV infection by 40%, say US and UK scientists.
People of African descent have a variation of the "DARC" gene which may interfere with their ability to fight HIV in its early stages.
The Cell Host and Microbe study says the gene accounts for millions of extra HIV cases in sub-Saharan Africa.
However, people with the gene appear to live longer with HIV than others.
While the differences in HIV prevalence in different parts of the world can be partly explained by different social conditions and sexual behaviour, scientists have long suspected that there may be genetic reasons why the virus is rife in certain communities.
Research at University College London and the University of Texas focused on the Duffy Antigen Receptor for Chemokines (DARC) gene.
The gene influences the levels of chemicals called chemokines, which play a role in the body's defences against viruses, and a variation is held by approximately 90% of Africans.
The origins of the variation are unclear, but it is thought to have evolved in response to widespread malaria outbreaks by offering protection against that disease.
The researchers did not use volunteers living in Africa, but analysed data from a 25-year study of Americans from different ethnic backgrounds with HIV.
They calculated that, after taking account of social and economic differences, people with the genetic variation were 40% more likely to be susceptible to the illness.
If the gene variant were not present in sub-Saharan Africa, they said, they would expect to see approximately an 11% lower burden of HIV in the region.
An estimated 24.5 million people are living with the disease there, and there are approximately 2 million deaths per year.
However, paradoxically, having the variant could mean that the disease, once caught, does not advance as quickly.
People who carry it survive on average an extra two years longer than those who do not, said the researchers.
The precise reasons for both the any extra vulnerability to HIV, and the increased lifespan of HIV-infected people, are not clear, with the scientists suggesting a "complex interplay" between levels of the protein expressed by DARC, levels of chemokines, and levels of the virus.
Professor Robin Weiss, from UCL, one of the researchers, said: "The big message here is that something that protected against malaria in the past is now leaving the host more susceptible to HIV."
Dr Ade Fakoya, from the International HIV/AIDS Alliance, said that the findings could explain why some countries had a much higher prevalence of the disease.
He said: "There has always been this myth that people in sub-Saharan Africa were more likely to get HIV because of differences in their sexual behaviour, or that they are more promiscuous.
"This shows that it's not that simple, and I think it will be an important message for education programmes in these areas."
People who carry a gene for the blood cell abnormality sickle cell trait are also known to be protected against malaria.