The HIV virus' ability to cause Aids is slowing, a study of more than 2,000 women in Africa has found.
The research suggests a quickly evolving and less virulent HIV is among a number of factors leading to what campaigners believe is a turning point in the deadly pandemic.
Philip Goulder, an Oxford University professor who led the new study, said: "Overall we are bringing down the ability of HIV to cause Aids so quickly."
The finding comes as annual figures show that - for the first time since the outbreak began 30 years ago - the number of new HIV infections is lower than the number of HIV-positive people being added to life-long Aids treatment.
Campaign group ONE said it showed health officials were "finally getting ahead of the disease", describing the milestone as "the beginning of the end of Aids.
However, it adds that "three major challenges" remain: insufficient resources (the charity reports a $3 billion shortfall in funds annually); increased risk for marginalised groups such as LGBT people, drug users and adolescent girls) and the "fragility of the process as a whole".
Watch Alok Jha's report:
Thirty-five million people currently have HIV worldwide, while around 40 million people have died from Aids since it began to spread in the 1980s.
The new study was conducted in Botswana and South Africa - both of which have been badly hit by the disease.
It was published this week in the journal Proceedings of the National Academy of Sciences (PNAS).
Yesterday, Prince Harry and a host of celebrities revealed secrets about themselves in a campaign to erase the stigma around HIV.
The #FeelNoShame campaign was conducted with Sentebale, a charity helping vulnerable children in the African nation of Lesotho.