Nearly 90 million Africans could be infected by HIV in the next 20 years if more is not done to combat the epidemic, the UN has warned.
The UN says the worst is yet to come with the Aids epidemic
Some 25 million Africans have HIV, which causes Aids, at present.
The world body estimates the next two decades could see 89 million new cases of the disease in Africa - or up to 10% of the continent's population.
The UN recommends a committed campaign against HIV/Aids - and $200bn (£105bn) of investment - to stem its spread.
At best, taking more action against HIV/Aids could save 16 million people from dying of the disease and a further 43 million people from contracting it, the UN says.
"Millions of new infections can be prevented if Africa and the rest of the world decide to tackle Aids as an exceptional crisis that has the potential to devastate entire societies and economies," said UNAids chief Peter Piot.
The UN report concludes that if millions of Africans are still being infected by HIV by 2025, "it will not be because there was no choice".
"It will be because, collectively, there was insufficient political will to change behaviour at all levels... and halt the forces driving the Aids epidemic in Africa."
The study, entitled Aids in Africa, was compiled over two years using more than 150 experts.
According to the BBC's UN correspondent, Susannah Price, it demonstrates the dramatic impact government policies could have on the spread of HIV and Aids in Africa.
The report offers three different models of how the disease could affect the continent in 20 years, based on how much money and effort is invested in fighting it.
The worst-case scenario, in which funding and policies stay as they are now, foresees a fourfold increase in the total number of people dying from Aids.
The disease has already affected millions of lives in Africa
The report also looks at two more positive outcomes.
In the best-case scenario, international aid flows to Africa are doubled, investment in health systems is increased and agriculture and education and treatment is dramatically improved.
The report says that even in this case the total number of deaths would continue to rise.
According to our correspondent, the UN offers hope that the effective use of resources could eventually end the Aids epidemic in Africa.
At the same time, it warns that current levels of action could see the disease bring the entire continent to its knees.
Simon Wright, of ActionAid, said: "We are being shown the future and unless we act now it is very stark.
"All countries must be more ambitious in fighting Aids, otherwise generations of Africans will be lost."