TORONTO (AP) – Bill Gates, whose philanthropic foundation has donated nearly $2 billion to fight AIDS, on Sunday called for countries to “dig deep” to halt the deadly virus that causes the disease.

Gates made the plea at a news conference ahead of a conference bringing together more than 24,000 scientists, activists, celebrities, HIV-positive people and humanitarians seeking to stem the disease that has killed 25 million people since the first case was reported a quarter of a century ago.

Bill and Melinda Gates, whose foundation has given $1.9 billion to support HIV/AIDS projects worldwide since 1995, were to deliver the keynote address to open the gathering. On Wednesday, the foundation, made a $500 million grant to an international fund that provides AIDS assistance in poor countries.

“Obviously the AIDS epidemic is going to require all actors, particularly governments, to dig deep and make this a high budgetary priority,” Gates said.

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 AIDS was now the foundation’s top priority.

The discovery of a microbicide or oral prevention drug to reduce HIV transmission could be “the next big breakthrough” in the fight against AIDS, particularly for women in the Third World, he said.

“A woman should never need her partner’s permission to save her own life,” Gates said. “There’s progress on these, but the pace has been too slow.”

Dozens of grandmothers from Africa left to rear their grandchildren after the deaths of their children to AIDS, marched through Toronto on Sunday with their Canadian counterparts, singing and dancing with pop star Alicia Keys.

An estimated 13 million children in sub-Saharan Africa have been orphaned by AIDS – more than all the children under 18 in Canada, Norway, Sweden, Denmark and Ireland combined, according to the Stephen Lewis Foundation.

“The grandmothers are the silent victims of this pandemic and it’s now I feel the silence has been broken and that it’s the time to be heard,” Keys said.

“We grandmothers deserve hope. Our children, like all children, deserve a future,” Kenyan grandmother Joyce Gichuna said.

Also attending the 16th International AIDS Conference were former President Bill Clinton and actor Richard Gere, as well as international AIDS experts.

The executive director of UNAIDS, Dr. Peter Piot ­- a microbiologist who launched the first international project on AIDS in the Democratic Republic of Congo in the 1980s – told a youth congress that older men hold the key to ending the scourge.

“The toughest job in HIV prevention that we have is to make older men change their behavior,” Piot said, referring to men who have multiple sex partners, then pass along HIV to their wives, girlfriends or boyfriends through unprotected sex.

“The future of this epidemic is in our hands,” he said. “As long as we don’t change our behavior and put girls and women at risk, again, it’s not going to work.”

This year marks the 25th anniversary of the first reported cases of human immunodeficiency virus, or HIV. Since the beginning of the pandemic, nearly 65 million people have been infected with HIV and AIDS has killed more than 25 million people.

Today, an estimated 40 million people are living with HIV, some 7 million more than the entire population of conference host, Canada.