Microsoft co-founder and philanthropist Bill Gates has said his foundation will invest another $5bn in Africa over the next five years.
Mr Gates delivered the Nelson Mandela Annual Lecture ahead of Mandela Day, when South Africans are encouraged to donate 67 minutes of their time to help others.
He is also in South Africa to attend a global Aids conference which starts on Monday.
Mr Gates said the Bill and Melinda Gates Foundation has already invested more than $9bn in Africa, with a major focus on health.
He warned that if the world does not come up with more creative ways to make HIV treatment and prevention accessible, "the hard-earned gains made against HIV in sub-Saharan Africa over the last 15 years could actually be reversed" as high birth rates continue.
He said Africa is the world's youngest continent in terms of demographics. "By 2050, 40% of the world's children will live on this continent," he added.
Africa had made notable progress in fighting Aids, with Mr Gates noting that the last time South Africa hosted the global conference 16 years ago, only a few thousand Africans were on HIV drugs. At the time, the drugs were too expensive for many in the region hardest hit by the epidemic.
South Africa now says half of its infected population of 6.8 million people is being treated.
Mr Gates noted that Nelson Mandela fought stigma over the disease by announcing publicly the death of his son from Aids in 2005.
The philanthropist's speech focused largely on youth and on how to achieve ambitious global development goals endorsed by countries last year.
In the only sharp words of his address, he said: "I get angry when I see that Africa is suffering the worst effects of climate change, although Africans had almost nothing to do with causing it."