Mutations in a single gene may have turned the AIDS virus from a fairly benign infection of monkeys and apes into a global pandemic that has killed more than 25 million people in 25 years, researchers said on Sunday.
The virus in humans appears to have lost a genetic characteristic that protected the immune system in apes and monkeys, the researchers say in the journal Cell.
Dr Beatrice Hahn of the University of Alabama at Birmingham had earlier shown that the human immune deficiency virus, or HIV, descended from a chimpanzee virus called simian immunodeficiency virus, or SIV.
Many species of monkeys, and chimps, are infected with various strains of SIV and it almost never causes disease.
But HIV destroys the human immune system, leading to AIDS acquired immune deficiency syndrome and usually death. There is no cure or vaccine.