By Martin Plaut BBC News

"This is a chilling statistic," says Professor Rodney Phillips of Oxford University, who carried out the analysis.

Aids is devastating countries across Southern Africa

Botswana has the highest instance of HIV infection in the world.

Over one in three of the population is infected. The countries in the rest of southern Africa are not much better off.

In Zimbabwe and South Africa the statistics are nearly as high.

Hopes of finding an effective vaccine in the near future are fading, says Professor Phillips.

'Mixed blessing'

Only a radical change in sexual behaviour will halt the epidemic.

Drug-resistant HIV could become a very severe problem for Africa Professor Phillips

Even a reduction in the costs of drugs could prove a mixed blessing.

Professor Phillips warns that they have to be taken for life.

If a patient ran out of drugs, or stopped taking them because he or she felt better, the results could be devastating.

The virus could develop resistance to the drugs, making the infection untreatable, he says.

AIDS IN AFRICA Africa's growing epidemic In Maps

"This virus develops resistance with immense speed.

"Then drug-resistant HIV could become a very severe problem for Africa," he says.

The scale of the Aids epidemic in southern Africa is hard to comprehend.

Fighting it will take all the resources and political will that can be mustered.