http://www.bbc.co.uk/news/health-30254697
every so often HIV infects someone with a particularly effective immune system.
"[Then] the virus is trapped between a rock and hard place, it can get flattened or make a change to survive and if it has to change then it will come with a cost," said Prof Philip Goulder, from the University of Oxford.
The "cost" is a reduced ability to replicate, which in turn makes the virus less infectious and means it takes longer to cause Aids.
This weakened virus is then spread to other people and a slow cycle of "watering-down" HIV begins.
The team showed this process happening in Africa by comparing Botswana, which has had an HIV problem for a long time, and South Africa where HIV arrived a decade later.
Prof Goulder told the BBC News website: "It is quite striking. You can see the ability to replicate is 10% lower in Botswana than South Africa and that's quite exciting.
"We are observing evolution happening in front of us and it is surprising how quickly the process is happening. ...