The United Nations has reduced its estimates of how many people are infected with HIV in 2007 from nearly 40m to 33m.
The majority of those with HIV are in Africa
Revised figures for India account for much of the decrease, experts say.
But the rate of new cases and mortality levels are declining, although figures still show that there are 6,800 new cases each day and over 5,700 deaths.
Africa has by far the most number of cases, while parts of Asia have the fastest growing rates of infections.
Some 22.5m in sub-Saharan Africa have HIV but the number of new cases - 1.7m a year - is a smaller increase than in previous years.
In Asia there are 4.9m people with the condition with Vietnam seeing a doubling of cases since 2000.
And the number of people living with HIV in Europe - including parts of Asia - has gone up from 1.25m in 2001 to about 2.4m, figures show.
UNAids executive director Dr Peter Piot said: "The improved data presents us with a clearer picture of the Aids epidemic, one that reveals both challenges and opportunities.
"Unquestionably, we are beginning to see a return on investment.
"But we must expand our efforts in order to significantly reduced the impact of Aids worldwide."
The figures show there were 2.5m new cases in 2007, down from a peak in the late 1990s when there was over 3m new infections a year.
The fall in annual deaths to 2.1m has been put down to wider access to antiretroviral drug treatments.
It means some 33.2m have HIV, down from 39.5m in 2006.
UNAids said the figures for 2006 were likely to be inaccurate after an intensive assessment exercise in India showed fewer cases than estimated. Other countries, including Nigeria, Mozambique and Zimbabwe, also had their figures reduced.
Experts say the true 2006 figure was likely to be about 32.7m.
Professor Brookmeyer, a US public health expert at the Johns Hopkins Bloomberg School of Public Health, said having accurate figures was important in combating HIV.
"More accurate estimates and trends will ultimately lead to improvements in the design and evaluation of prevention programmes."
But Michael Weinstein, of the US group, the Aids Healthcare Foundation, questioned how accurate the latest figures were.
"Because the vast majority of people who are infected with HIV don't know it, there is actually no way to know if this new WHO figures is any more reliable than the previous estimation."
And the Terrence Higgins Trust said more needed to be done to tackle HIV in the UK as recent figures suggest the rate of new cases is rising.
Chief executive Nick Partridge said: "There is too little local investment and too little national focus on HIV which means our prevention efforts are not enough to keep it at bay.
"We need to refocus on HIV in the UK and reinvest in prevention, rather than stand by and watch the spiralling costs of treatment as ever more people test HIV positive."
HIV PREVALENCE IN SUB-SAHARAN AFRICA 2003-7
22.5m (68%) out of 33.2m people globally with HIV
61% are women
More than three-quarters of all Aids-related deaths in 2007
Southern Africa worst-affected in the region - prevalence above 15% in eight countries
South Africa has more HIV infections than any other country in the world
But HIV prevalence in most of region has reached or is approaching plateau
Nigeria: 'No data' because the country does not conduct a nationwide population-based survey including an HIV component. New information from other countries with similar epidemics has allowed the revised methodology to be applied to re-estimate Nigeria's epidemic. This has shown a lower number of people living with HIV than previous estimates indicated. For more information, read the full report:
Most computers will open this document automatically, but you may need Adobe Reader