The rapid evolution of HIV is slowing its ability to cause AIDS, according to a study of more than 2,000 women in Botswana and South Africa. Scientists said the research suggests a less-virulent HIV could be one of several factors contributing to a turning of the deadly pandemic, eventually leading to the end of AIDS. “Overall, we are bringing down the ability of HIV to cause AIDS so quickly,” said Philip Goulder, a professor at Oxford University who led the study. “But it would be overstating it to say HIV has lost its potency.” Some 35 million people have HIV, and AIDS has killed around 40 million people. But campaigners noted that for the first time, the annual number of new HIV infections is lower than the number of HIV-positive people being added to those receiving treatment.