No one imagined 25 years ago that AIDS would become the deadliest epidemic in history. Since June 5, 1981, HIV, the virus that causes AIDS, has killed more than 25 million people, infected 40 million others and left a legacy of unspeakable loss, hardship, fear and despair.

Its spread was hastened by ignorance, prejudice, denial and the freedoms of the sexual revolution. Along the way from oddity to pandemic, AIDS changed they way people live and love.

Slowed but unchecked, the epidemic's relentless march has established footholds in the world's most populous countries. Advances in medicine and prevention that have made the disease manageable in the developed world haven't reach the rest.

In the worst case, sub-Saharan Africa, it has been devastating. And the next 25 years of AIDS promise to be deadlier than the first.

AIDS could kill 31 million people in India and 18 million in China by 2025, according to projections by UN population researchers. By then in Africa, where AIDS likely began and where the virus has wrought the most devastation, researchers said the toll could reach 100 million.

"It is the worst and deadliest epidemic that humankind has ever experienced," Mark Stirling, the director of East and Southern Africa for UNAIDS, said in an interview.

More effective medicines, better access to treatment and improved prevention in the last few years have started to lower the grim projections. But even if new infections stopped immediately, additional African deaths alone would exceed 40 million, Stirling said.

"We will be grappling with AIDS for the next 10, 20, 30, 50 years," he said.

Efforts to find an effective vaccine have failed dismally, so far. The International AIDS Vaccine Initiative says 30 are being tested in small-scale trials. More money and more efforts are being poured into prevention campaigns but the efforts are uneven. Success varies widely from region to region, country to country.

Still, science offers some promise. In highly developed countries, cocktails of powerful antiretroviral drugs have largely altered the AIDS prognosis from certain death to a manageable chronic illness.