While the Microsoft tycoon applauded efforts to get more people worldwide on antiretroviral drugs -- and said his foundation funds both research and care -- he noted more is needed to stamp out the deadly disease.

"No one should think that we have got the tools yet. We will get the tools but only if we stay the course in terms of the scientific investments," Gates told the International AIDS Conference in the US capital.

His Bill & Melinda Gates Foundation has committed more than $2.5 billion in HIV grants to organizations around the world, and has also committed more than $1.4 billion to the Global Fund to Fight AIDS, Tuberculosis and Malaria.

Gates spoke to the conference, the world's largest gathering of AIDS scientists and advocates, as part of a panel on how to improve efficiency in funding of the three-decade-old disease that has killed 30 million people.

Gates said research toward a vaccine is "very exciting" but noted that even if a vaccine were introduced it would take time before the effects would be seen in the larger population.

"If you get a vaccine say in 10 years then the number of people you need to put on treatment is reduced in about 18 years," he said.

"Unfortunately there is this unbelievable lag time that comes out of that."

More than eight million people in low and middle income countries were on antiretroviral treatment in 2011 according to a UNAIDS report released last week, making up about half of those in need worldwide.

But Gates warned that no amount of funding can come up with enough money to treat everyone infected.

"It is clear that even if you take the most efficient way of doing this work -- the number of people who will eventually need to be on treatment, the amount of money we have is not enough to treat those people," he said.

"The world will make a decision how much those lives matter. And we are in a period of incredible uncertainty right now," he added.

"Just the uncertainty alone creates a certain instability."

Gates urged continued involvement by the AIDS community and reiterated the importance for nations and donors to support research, but also expressed support for ongoing treatment initiatives in the meantime.

"Now if somebody could cure AIDS -- which unfortunately that's very much a long shot," he said.

"There are people who are working it but... it is not in the cards at least any time soon, that is why this treatment imperative is so dramatic."

But that includes having unprotected sex in a country where 14 percent of the population has HIV.

"I am scared of contracting HIV through my sugar daddy, because he can go for any girl he wants," 17-year-old Mzungu told AFP.

She calls herself a "secret second wife" to her lover, a married man aged 55. She once turned to prostitution to support her child, but says her secret life now is better.

"I have no regrets because he has bought me a small house and a car," she said.

AIDS experts say cross-generational sex, especially among older men and younger women, is one of the reasons that African women bear a greater burden of HIV infections, about 60 percent of cases in southern Africa.

"It's one of the means for HIV transmission, especially men that have their spouses but they go out with these younger ladies," said Linda Chongo, advocacy officer of Mozambique's National Network of AIDS Service Organisations.

"It is quite difficult to negotiate safe sex when you are already in a lower position. As money rules, the person with the money will be the one who will impose the rules to be taken," she said.

"Some of the reason is poverty but if you look into Africa you will see that poverty has always been there, but our grandmothers and mothers didn't behave the way we are behaving now," Chongo said.

Stuart Chuka, a coordinator of Malawi's AIDS treatment programme, said that even women who know the dangers of HIV find themselves in relationships with older men.

"One of the reasons is that the young women do not have the capacity more especially because of finances," to provide for themselves, he said.

"They should be able to negotiate for safer sex, they should be able to be comfortable to say no," he said.

-- 'We are suffering more' --

In Zimbabwe, these long-term extra-marital affairs between older men and young women are known as "small houses" -- as opposed to the "big house" where a man's wife lives.

"As part of our on-going campaign to prevent new infections, we are designing posters to discourage age-mixing in relationships," said Beauty Nyamwanza, a programme officer at Zimbabwe's National AIDS Council.

The health ministry has already raised billboards showing women university graduates who testify about how they have made it without the support of a "sugar daddy".

Such campaigns appear to be paying off.

"There has been a lot of progress, the number of new infections is declining in the region," said Rick Olson, an HIV specialist at UNICEF, the UN children's agency.

"The number of condoms purchased by governments have also grown," he added.

But younger women still need support to give them the confidence to demand condoms, he added.

"When partners are older, it's hard to negotiate persistent condom use," he said.

"It's for people to understand that the risk in a cross generational relationship is that young girls lack the power to negotiate safe sex."

In South Africa, an increase in teenage pregnancies in eastern KwaZulu-Natal was blamed on older men seeking out younger women. The provincial government has set up 89 huge billboards to highlight the dangers of sex with older men, while creating support groups for young women to resist such relationships.

South Africa is also launching clinical trials for a vaginal ring which releases ARVs into the body, warding off infection.

Sibongile Molefe, a 41-year-old women with HIV, said the efforts are welcome.