A global conference on HIV/Aids has opened in Mexico City, a quarter of a century after the disease first became widely known.Figures released ahead of the meeting reveal that the number of people with the condition around the world has gone down slightly overall.However, infection rates are still rising in some countries and access to the right treatment is also an issue.Across the world 33 million people are affected by the syndrome.The six-day conference was preceded by an awareness march, a photo exhibition and other events.About 20,000 scientists, government officials and campaigners are in Mexico City for the event.Worrying spreadFormer US President Bill Clinton is due to attend on Monday.Since Aids first became known, 25 million people have died.In one positive development, US President George W Bush recently won backing to triple US spending on combating the syndrome.But in some countries like Russia and China, and even Germany and the UK, the rates of infection are rising, the BBC's Duncan Kennedy reports from Mexico City.In the US, better detection methods have just shown the figures there have been underestimated by about 30%.And in Africa, home to 70% of cases, access to the right drugs is improving but there are not enough health care workers to administer them.There are concerns, too about the human rights of sufferers who are often too scared to seek treatment.It all means that the 17th international HIV/Aids conference has much to discuss, our correspondent says.