"The largest generation in history is entering the age where they are most at risk of HIV‚" billionaire philanthropist Bill Gates told the 21st international Aids conference in Durban on Wednesday.He said that speaking to young girls he met at a clinic in Durban this week was "very sobering". Girls between 15 and 24 are most of risk at getting HIV in South Africa.The bulging of the youth population‚ who are most likely to contract HIV‚ meant the numbers of HIV infections could continue to increase‚ he warned.Gates said: "In 1990 on the (African) continent there were 94 million people between the ages of 15 and 24. Already that number has doubled. By 2030 there will be more than 200 million young people… The vulnerable age group will be three times as large in 2030 as it was back in 1990. What that means is pretty clear.""If we have constant incidence rates (new infections)... If we only do as well as we have been doing‚ the number of people getting HIV will go up even beyond its previous peak. We have to do a lot more now to.... (achieve a) downward path of people who get infected."He said we have to reach "very ambitious treatment goals and very ambitious prevention goals" to turn the tide against Aids.Gates gave South Africa a nod for planning to roll out HIV preventative therapy‚ the pill Truvada‚ to HIV negative sex workers.He said circumcision was the single best prevention tool that the world had at the moment and he was glad donors were continuing to fund programmes to circumcise men.There have been 12 million men already circumcised‚ he said. But more had to be done and more men snipped.He also said‚ in order to improve treatment of HIV‚ "we have to reach the 16 million positive people who have not yet been tested and diagnosed. Testing has to be made more accessible‚ including self testing".But prevention as it stood currently was not good enough and research had to carry on‚ which was why his foundation budgeted so much money towards research and development.While he was optimistic that the battle against Aids could be won‚ he said it was "proving more difficult than expected".- TMG Digital/The Times..