Last week, I stood in the blinding sunshine outside the United Nations building in New York, watching cars disgorge eunuchs, activists and bureaucrats - all the usual suspects for another “UN high-level meeting on Aids”. Besides swelling profits of the New York hotel trade with money that could have been used to buy condoms and clean needles, what did this meeting achieve? Not a lot, I was prepared to report, but in fact the UN has managed to take a step backwards.

For all the talk of a “global pandemic”, there are two completely separate HIV epidemics in the world. One is in parts of Africa, where HIV is spread by unprotected sex between men and women who have more than one steady partner. Governments - such as Uganda's, with its “zero grazing” approach to fidelity - that recognised the perils of the custom of having concurrent sexual partners confined the epidemic. Most didn't. The result of the neglect is that in some countries up to two in five adults are infected with a fatal virus.

The second epidemic covers the rest of the globe. Nine out of ten humans (and three in ten of those infected with HIV) live in countries where the virus is spread mostly when people buy and sell sex, when they shoot up drugs, and when men have anal sex with lots of other men. Only a minority do these things in any country, but that still adds up to several million people worldwide. We know how to prevent HIV in these populations, and we have known for years that in Asia, the Americas, Europe, North Africa and the Middle East, if you do that prevention well, HIV won't spread farther. Even if you don't control HIV in these populations, it won't go all that much farther.

If we don't recognise this, we will never effectively prevent the spread of HIV. But a lot of UN agencies, governments and even Aids activists don't want to recognise it. Governments don't want to because it would mean recognising that if they want to deal with HIV they have to spend money on services for junkies, sex workers and gay men - groups that don't top the popularity stakes with voters. Ironically, they will happily fund treatments for these people with expensive medicines once they do get sick. That is more acceptable to voters than to give cheap condoms and needles to prevent them getting infected in the first place.

Activists don't want to recognise it because they fear (with some justification) that if HIV is seen as a disease of junkies, prostitutes and gays, they will lose both public support and money for prevention, as well as increasing prejudice against groups that are already vilified. A couple of UN agencies - the ones that actually spend money on doing things for those politically sensitive groups - share that fear. But most have a much greater fear of getting pushed away from the Aids funding trough. Since that trough now holds around $12 billion (£6 billion) a year and rising, it's a lot to lose out on.

If the UN were to recognise that in most of the world Aids is not a multisectoral problem that threatens security or undermines economic development, then the agencies that work with agriculture or children would have to deal with drugs, commercial and homosexual sex, or get out. Since they want the money but not the inconvenience of dealing with such dirty issues, there is a conspiracy of silence.

If you dig into the entrails of the epidemiology chapters of the WHO/ UNAids reports, you will find that the UN's public health experts acknowledge that HIV is never going to rage through other continents the way it has raged through Africa. But the numbers are usually sandwiched between “HIV affects us all” rhetoric that panders to the interests of everything but the truth.

That is why it was so surprising that Kevin de Cock, the head of the World Health Organisation's HIV division, told a British newspaper the truth last week. He was quoted as saying “It is very unlikely there will be a heterosexual epidemic in other countries [outside Africa]. Ten years ago a lot of people were saying there would be a generalised epidemic in Asia - China was the big worry with its huge population. That doesn't look likely.”

That Dr de Cock, a respected scientist, said this just before the Aids circus at the UN was surely a great sign. Maybe the worthies in attendance would agree to focus prevention outside Africa on the people who need it most, instead of throwing away money on multisectoral planning jamborees and programmes for schoolgirls.

The UN even planned a session on how to find the political will to do exactly that, chaired by the President of El Salvador. (Sadly he could not summon up the political will to talk about how gay and commercial sex is driving the epidemic in El Salvador, and pulled out.) But still, at least the WHO had finally told it like it is. That must certainly make it harder for other UN agencies, grouped under UNaids, to defend their institutional interests against scientific evidence and the rational use of taxpayers' money.

It was not to last. Before the meeting was even over, the WHO and UNAids came out with a press release, signed by Dr de Cock, billed as a “correction” to the newspaper report. But it didn't point out any errors of fact, nor did it suggest that he had been misquoted. Instead, it claimed that HIV is a heterosexual disease that affects us all.

Just as it looked as though it might make a step forward, the UN has jumped back into a refusal to acknowledge the truth about Aids. Its stance will defend the jobs and budgets for a while, perhaps. But unless a better job is done of preventing the epidemic, we the taxpayer will tire of refilling the Aids funding trough. And we can't do a better job of preventing HIV if we refuse to be honest about where it's spreading.

Dr Elizabeth Pisani is an epidemiologist who has worked for more than a decade as a consultant to UNAids and the WHO and is the author of The Wisdom of Whores: Bureaucrats, Brothels and the Business of Aids.

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