The UN has revised downwards its global estimate of the numbers infected with HIV/Aids, as a result of better information and understanding of the disease. Last year UNAids and the World Health Organisation (WHO) said 39.5 million people were infected with HIV and would need life-saving treatment now, or in the near future. But today the UNAids annual report says the true figure is 33.2 million.

The substantial drop in numbers will provoke critics of the UN, some of whom have accused its Aids-combating arm of artificially inflating the scale of the pandemic in order to attract more funds.

But the UN says the revision is very largely due to a better assessment of the scale of the epidemic in India. Similar improvements in data collection have led to lower figures also for Angola, Kenya, Mozambique, Nigeria and Zimbabwe.

But the smaller numbers should not lead to complacency, said Kevin de Cock, director of HIV/Aids at the WHO. "This remains the leading infectious disease challenge to global health," he said. "We have to recognise the very long-term nature of the HIV pandemic. We're facing decades of this problem. Of the 33.2 million infected, some of them require treatment now and all of them will in time. The qualitative implications have changed very little.

"We are very well aware of Aids fatigue and the situation in the industrialised world where the advent of treatment has dampened enthusiasm for prevention."

Paul De Lay, UNAids director of evidence, monitoring and policy, warned that new infections hit 2.5m last year, with 2.1m deaths: "We do not have a curative intervention," he said. "We have repeated evidence from countries that as we back away and become less vigilant and our programmes start to diminish, we are seeing a return of the epidemic."

In the USA, the UK, Germany and also in Uganda, which was once lauded for bringing down its HIV rates, prevalence was rising again.

Treatment programmes in sub-Saharan Africa, where 1.5 million people are now on antiretroviral drugs, are having results, the UNAids report says, but death rates remain high because of the large numbers who cannot get the drugs. There is also evidence of a reduction in high-risk behaviour in some countries, such as having many sexual partners, as a result of education efforts - the report cites Botswana, Cameroon, Chad, Haiti, Kenya, Malawi, Togo, Zambia and Zimbabwe.