The revised figures, which were the result of much more sophisticated sampling techniques, indicate that the number of new infections peaked in 1998 and the number of deaths peaked in 2005.
The new analysis shows that the total number of people living with HIV has been gradually increasing, but at a slower rate than in the past.
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Hints of those trends were present in the older estimates, but at much greater numbers.
UNAIDS estimated in a report to be issued today that about 2.5 million people will be infected with the AIDS virus, called HIV, this year -- a 40% drop from the 2006 estimate.
The report also says that about 33 million people worldwide are infected with the virus, compared with last year's estimate of almost 40 million.
Reports over the last decade or longer have portrayed a disease spiraling out of control, but improved methods of counting people with AIDS have unveiled a different picture.
"For the first time, we are seeing a decline in global AIDS deaths," said Dr. Kevin De Cock, director of the AIDS department at the World Health Organization.
UNAIDS Executive Director Dr. Peter Piot said the new estimates also reflected improved treatment rates and changes in sexual behavior in some affected regions of the world.
"These improved data present us with a clear picture of the AIDS epidemic," he said. "Unquestionably, we are beginning to see a return on investment."
The data represent some of the first good news in the battle to control the pandemic, particularly coming after recent reports indicating that promising HIV vaccines are ineffective and perhaps even dangerous.
The numbers have been highly politicized because they are used to govern the distribution of the billions of dollars in aid that is being poured into the problem by industrialized countries -- an estimated $10 billion this year.
Some critics viewed the changed estimates as a remarkable admission by world health authorities that they had made a mistake.
Dr. James Chin of UC Berkeley, a former WHO AIDS expert who has been tracking the disease since it first emerged in California in the 1980s, has been arguing for years that the UNAIDS figures have been inflated.
"It's getting closer to what it ought to be, but it's still high," he said. "It seemed to me that high-rise house of numbers had to crumble."
Chin estimated the total number of cases worldwide at 20 million to 30 million.
UNAIDS has "been overemphasizing and exaggerating numbers in an effort to get more and more money," Chin said. "A lot of people say the ends justify the means. It's going to come back and bite you when the real numbers appear."
Dr. Paul De Lay of UNAIDS said that he considered it "absurd" to think his agency would exaggerate the data.
"It would be technically impossible to somehow rig the numbers," he said.