Toronto ? Bill and Melinda Gates, whose foundation has contributed $1.9 billion to fight AIDS, said Sunday that the search for HIV prevention drugs that would empower women could be the “next big breakthrough” in combating the disease.

The couple joined more than 24,000 scientists, activists, celebrities, HIV-positive people and humanitarians from 132 countries for a conference on how to combat the disease that has killed 25 million people since the first case was reported a quarter of a century ago.

Bill Gates, who recently announced he would step down from his day-to-day duties at Microsoft Corp. and devote more time to philanthropy, said the search for a vaccine to prevent the virus that causes AIDS was now the foundation’s top priority.

“We want to call on everyone here and around the world to help speed up what we hope will be the next big breakthrough in the fight against AIDS – the discovery of a microbicide or an oral prevention drug that can block the transmission of HIV,” Gates said at the opening ceremony of the 16th International AIDS Conference. “This could mark a turning point in the epidemic, and we have to make it an urgent priority,” he added.

Microbicides are gels or creams women can use to block infections and disease. Sixteen microbicides are being clinically evaluated; five are in major advanced studies.

The Gateses called for greater advocacy to break the stigma of AIDS for women in impoverished nations who typically have little say over their own sex lives or health.

“We need tools that will allow women to protect themselves,” Bill Gates said. “This is true whether the woman is a faithful married mother of small children, or a sex worker trying to scrape out a living in a slum. No matter where she lives, who she is, or what she does – a woman should never need her partner’s permission to save her own life.”

Since the beginning of the AIDS pandemic in 1981, nearly 65 million people have been infected with HIV; AIDS has killed more than 25 million people.