The rate at which people are infected with HIV may have peaked in the late 1990s, according to a UNAids report.
Two-thirds of people in Asia with HIV are in India
It found the incidence of new HIV infections appears to have stabilised for the first time in 25 years.
UNAids said improved funding and access to drugs appeared to be producing results - but said HIV remained "an exceptional threat".
It warned the infection rate was still rising in some countries, and record numbers now live with the virus.
The agency, which surveyed 126 countries, estimated that 38.6m people are living with HIV worldwide.
It found that approximately 4.1m people were newly infected with the virus during 2005, and approximately 2.8m people died of Aids-related illnesses during the year.
The report warns young people and children are increasingly affected by the epidemic, and efforts to protect these groups are not keeping pace.
It found some countries were doing well on treating people with HIV, but poorly on HIV prevention efforts, while in others the opposite was true.
The report said there was still a need for improved planning, sustained leadership and reliable long-term funding - with current levels of spending still below what was required.
The report was released on the eve of a UN General Assembly meeting on Aids in New York to review progress in the five years since a major international declaration setting goals for tackling the virus.
Dr Peter Piot, UNAids executive director said efforts to combat HIV appeared to be starting to produce results.
He said: "The actions we take from here are particularly important, as we know with increasing certainty where and how HIV is moving, as well as how to slow the epidemic and reduce its impact."
Africa worst hit
Sub-Saharan Africa remains the epicentre of the Aids pandemic, with two-thirds of all people living with HIV coming from the region.
Two million people died of Aids in the region last year and there were 2.7m new infections.
However, the numbers affected in Eastern Europe and Asia continue to rise.
India, Ukraine and the Russian Federation are particularly badly affected.
India overtook South Africa as the world's worst-affected country in terms of the absolute number of people with HIV - although not as a proportion of the population.
The report says the "3x5" target to get anti-retroviral therapy (ART) to three million people by 2005 was missed, but access to treatment has significantly improved.
In 2001, just 240,000 people in developing countries had access to ART.
By 2005, that had risen to 1.3m.
However, UNAids said that means only one in five people around the world who need the drugs gets them.
The report calls for the price of ART drugs to be cut to widen access to treatment.
The report found HIV prevention programmes are failing to reach those most at risk.
The UN's Declaration of Commitment on HIV/Aids said 90% of young people should know about the disease by 2005.
But surveys suggest fewer than 50% have comprehensive knowledge.
Just 9% of men who have sex with men received any type of HIV prevention advice, and only 20% of intravenous drug users.
However, there has been some success. Six out of 11 African countries reported declines of at least 25% in HIV prevalence among 15-24-year-olds in capital cities.
Rates of sex among young people declined in nine of 14 sub-Saharan countries.
And condom use with a non-regular partner increased in eight of 11 countries in the same region.
The report also called for a significant increase in funding.
It said $8.3bn was made available in 2005, more than five times the funding available in 2001.
However, it warns that over $20bn will be needed annually from 2008.
UNaids also called for world leaders to fulfil their commitment made at the 2005 G8 Summit to achieve universal access to HIV treatment by 2010.
Deborah Jack, of the UK's National Aids Trust, said: "Behind the shocking statistics there has been progress in improving access to drugs and in rolling out prevention programmes in developing countries.
"This shows that interventions can and do work. But such efforts must be accompanied by the promotion of human rights."
Nick Partridge, of the Terrence Higgins Trust, said: "Governments and businesses across the world must continue to do more to increase access to anti-retroviral treatments, good sex education and condoms, which we know are vital in reducing the spread of HIV."