many morning, thousands laced up for the annual aid walk as diane guerazzi reports the walk came at the same time new research is beginning new hope in the fight against aids. >> reporter: it was congratulations all around as 25,000 people took part in the san francisco aids walk. it's been held 24 years, but this year, even though most walkers didn't know it, they had something extra to cheer about. >> jai! about. >> reporter: a new study was presented at an international aids conference in vienna. researchers found as patients went on aids drug and lowered the amount of virus in their system, the chance that they would spread the disease flumetted. >> every time there is a study like this, we know more and more about the science that can priest new hiv infection. >> reporter: the study found that the number of new infections has been cut in half in british columbia. that is encouraging four aids activists and volunteers here at the walk, but this man who is hiv-positive worries that such studies give a false sense of security, especially among young people engamed in a new trend calle