JOHANNESBURG - Microsoft co-founder and philanthropist Bill Gates says his foundation will invest another $5 billion in Africa over the next five years.

Gates on Sunday 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.

Gates is also attending a global AIDS conference that starts Monday in South Africa.

Gates said the Bill & Melinda Gates Foundation has already invested more than $9 billion in Africa. Health is a major focus.

Gates warned that if the world doesn't act to 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