THERE WAS a time, in the early 1980s, when every weekend brought another funeral for New York's gay community. Back then, Elizabeth Pisani was a gap- year student working for an advertising company in the city, and this was her first encounter with the disease in which she would later specialise as an epidemiologist. "Have you ever seen a person die of Aids?" she asks. I have not. Her point is that in most countries outside sub-Saharan Africa, the development of anti-retroviral drugs has meant that people no longer die of the disease in the same dramatic, visible way. Though there are still some Aids-related deaths, they are not so evident. "There aren't all these people with these whole clusters of symptoms that are in your face. It used to be that someone would shuffle into a bar with black blotches all over their face and you'd think, Oh, there's another one.' There were 10 years where it was a very visible thing and it has ceased to be so in developed countries."

In almost all ways this is a good thing. The only problem, says Pisani, is that because people no longer come into contact with others in such an alarming condition, they are less terrified of the disease. Less fear means less assiduous attention to not contracting HIV. This much is illustrated by the patterns in sexually transmitted diseases in San Francisco's gay community. As Pisani explains, these men were once the "Formula One drivers of condom adoption", whose use surged from zero to around 70% by 1985 and stayed that way until 1994. Then, with the advent of good anti-retroviral treatment, condom use plummeted. Incidences of other sexually transmitted diseases rose, and by 2001 research showed that half of all HIV-positive men had dispensed with condoms. Similar patterns were recorded in London, Amsterdam and Vancouver. Pisani writes: "Treatment makes HIV much, much less scary, because it makes it less fatal. There are few cadaverous people around, fewer funerals to go to. With treatment, people who were at death's door leap up and march back to the office and the nightclub."

In her controversial book, The Wisdom Of Whores: Bureaucrats, Brothels And The Business Of Aids, Pisani, a former UNAIDS and World Health Organisation employee, uses this piece of research to illustrate a particular point. Talking in her London home, she explains how the conventional wisdom in what she calls "the HIV industry" has been that treatment is a form of prevention. When an HIV positive person is on anti-retroviral drugs, their viral load is usually brought down to such a low level that the likelihood of them infecting someone else is small.

This may sound like a neat fusion of treatment and prevention, but the problem, argues Pisani, is that familiarity with this fact results in a reduction of condom use since people assume that anyone with Aids is likely to be on treatment and therefore less infectious. As a result, a window of opportunity is opened for the virus. During the period between contracting Aids and being tested - often around four to five months - people are highly infectious.

The second problem is a more general one. "What is the viral load of someone with Aids who is not taking Aids medicine?" asks Pisani. "It shoots up for a while, maybe a year, possibly two. Then it drops to zero, because the person dies. The person on therapy can carry on for years and years - we don't know how long yet, because the drugs are too new. On the whole they'll be in pretty good shape. Good enough to go out dancing, to sell sex, to party with friends and shoot up drugs. They'll probably have the odd spike in viral load now and then even when they are on medication, if they get the clap or some other infection. Someone whose viral load is low with the occasional spike and who is getting laid is certainly more likely to transmit HIV than someone who is dead."

At first glance, it looks as if Pisani is pushing a case against treatment. Let them all die, she seems to be saying. But this is not her argument. Rather, what she is trying to emphasise is that the more treatment we provide, the more we need prevention. With these dangers in mind, people on anti-retrovirals should be encouraged, perhaps even compelled, to go into prevention counselling programmes and those in communities at risk should be hooked up to advice. The condom and the clean needle still need to lie at the heart of the matter. Indeed, The Wisdom Of Whores can be read as a manifesto for a back-to-basics approach. Politically, treatment is a vote-winning topic. Prevention, however, is a subject tainted by the squeamishness and moral sensitivity that surround the discussion of sex and drugs. Most politicians and other public figures would rather avoid talking about how we exchange bodily fluids and who we exchange them with.

In Pisani's view, the billions of pounds spent on combating the spread of Aids over the past decade have been misdirected. Her book title, The Wisdom Of Whores, hints at the idea that in what she calls the Aids industry, "we are all whores", doing whatever it takes to get money. Too much of that funding, she believes, has not been targeted at the people at risk or the behaviours that spread the disease: the sex workers, gay community, and anyone living in much of sub-Saharan Africa, where in some countries, for example, almost half of adults are infected. "According to some, HIV is a human-rights problem, or it's a development problem, or it's a gender issue," she says. "But actually HIV is a virus so what do you need to spread a virus? You need someone who's infected. You need someone who's not infected. You need them to swap body fluids and you need one to be highly infectious or one to be susceptible for there to be a new infection. HIV is not very infectious. If you want to stop the spread of HIV you need to reduce the likelihood of at least one of those things."

In her time working around the world in Thailand, Indonesia, China and Africa, where she learned much of what she knows from talking to those in the sex industry, she has seen plenty of well-meaning projects which are doomed to ineffectiveness because they are aimed at people whose risk of contracting HIV is extremely low. Pisani argues that funding has been targeted towards particular categories of people deemed to be "at-risk", including those defined by the tick-boxes "youth" or "migrant worker". Funding for these groups is often relatively easy to get despite the fact that only particular youths and certain migrant workers are at risk. "You're not at risk of HIV if you're a young person," explains Pisani. "You're at risk if you're a young person who takes drugs, or if you have sex with a lot of other young people. It's about very specific behaviours. So we need to target our money. One of the great obstacles to that is that almost none of the big institutions really have those specific behaviours as their primary mandate."

Pisani has the epidemiologist and statistician's disdain for political correctness. She does not shy from the uncomfortable truths her figures throw up. Why does HIV/Aids pose a greater problem in East and Southern Africa than elsewhere in the world? It has to do with sex, says Pisani, who in the past has been accused of racism. "If you ask me why is there more aids in Africa and I say because Africans f*** more, what's your reaction?" she asks.

In fact, though, Pisani's point is not that Africans have more sex; rather it's that patterns of sexual behaviour play a part in transmission. In the African countries where Aids is the biggest problem, the networks of sexual contacts create the kind of web that allows the virus to move rapidly through the population. Research has shown that people in industrialised countries tend to have more sexual partners over a lifetime than Africans, but that in many African countries, men and women are more likely to be sleeping with more than one partner at once. As a result, a web rather than chain pattern of contacts is established, through which the virus spreads more widely.

Pisani quotes a Ugandan friend and colleague who believes this is a legacy of polygamy in her country. The book draws attention to the vastly different sexual cultures which exist throughout the world: not just in terms of how people do it, but who they do it with and which gender is permitted to do what. Preventing Aids, therefore, is not just a matter of dropping packages of clean needles and condoms from the sky. What has to be understood, in order to persuade or tie people in to prevention, is the sexual landscape of a place.

In Botswana, which has a 40% infection level, or South Africa, where 19% of those aged 18-49 are infected, it is highly risky for an HIV negative person to have unprotected sex with almost anyone. As Pisani writes: "For the same amount of sleeping around, you now have a greater risk of getting infected with HIV if you use a condom every single time you have sex in Swaziland than if you never use a condom at all in China."

One of the additional factors in the dissemination of the disease is what Pisani calls "intergenerational sex". Research has found that young girls in some African countries sometimes take on older, richer boyfriends before they get married. In Kenya they call them "Three Cs Boyfriends" because they provide cash, cars and cellphones. Thus young women are more likely to be infected than young men. In fact, for a young man in the heavily infected countries of Africa, almost the most dangerous thing he can do is to get married. Pisani explains: "In nearly every African country where we have data, unmarried girls are more likely to be infected with HIV than unmarried men. Once a man marries, though, he'll have sex with his wife a lot more frequently, and condoms - if they were ever part of the picture - will evaporate."

For Pisani it is obvious that countries such as Uganda and Senegal, where leaders and governments have been willing to recognise their Aids problem and its sexual nature, now have relatively low HIV levels. By contrast, countries have fared less well where the sex issue was in some way denied: for instance, in South Africa, where health minister Dr Manto Tshabalala-Msimang recommended eating beetroot as an alternative to anti-retrovirals, or Mozambique, where Archbishop Francisco Chimoio suggested that anti-retrovirals were infected with HIV "in order to quickly finish off the African people".

In fact, political sensitivities have affected the efficacy of HIV/Aids prevention and treatment. Pisani questions, for example, the wisdom of describing Aids as a disease of poverty and gender inequality. "If HIV is spread by poverty and gender inequality," she says, "how come countries that have plenty of both - such as Bangladesh - have virtually no HIV? How come South Africa and Botswana, which have the highest female literacy and per capita incomes in Africa, are awash with HIV?"

The history of Aids campaigning in the West - rooted, as it was, in the gay activist community - also affected the way the disease is now discussed. "HIV struck at the heart of the gay community in rich countries," says Pisani, "and no-one wanted to hear about it." Gay rights campaigners, who took up the issue, "shaped the way we approach the disease," she adds, and "there is an upside and a downside of that response. Part of the downside is that we've taken some of those things that really made sense in that context and translated them to other contexts where, in my opinion, they make less sense."

One of the orthodoxies passed down from these early years is that HIV testing exposes people to stigma so "routine testing is dubious and compulsory testing is evil". For many years, Pisani was a believer in this principle, yet she is now persuaded that involuntary testing can be the right thing since she saw how effective it was when teamed with the offer of free treatment packages in China.

"When I first heard about mass testing in China," Pisani writes, "I reacted with all the predictable knee-jerk comments about a violation of ethics. But now that we can do something useful for those who are infected, testing the people most likely to become infected is beginning to make sense. Could it lead to people being outed, being stigmatised? Yes. But untreated Aids has a way of outing people anyway so you get the stigma and the avoidably early death."

The story of HIV is also the story of the evolution of public health. Disease can be viewed either as a suffering of an individual or a wider public issue of a parasite on our population, and there is a tension between these perspectives. It is there, for instance, when we talk about vaccines for our children. Pisani, who has worked in the counting house of public health for the last 15 years, feels that "Aids focused people's attention on the rights of the people living with the infectious disease. This was a new departure in public health, and a welcome change. But the fear of violating people's perceived rights overrode many otherwise routine principles of public health."

She is not the only researcher who believes we may need to rethink our attitudes towards public health. In his book, Aids In The 21st Century: Disease And Globalisation, professor Tony Barnett states: "We must turn away from the excessive individualism of the final decades of the 20th century to a recognition of the ways in which our individuality depends on common undertakings for the common good. Health and wellbeing are human rights; they are also public goods."

Pisani wants to see sweeping changes in the way the prevention and treatment of Aids is approached, including the universal availability of free clean needles and US charitable aid that is free of restrictions tying it to abstinence programmes or US drug suppliers. Above all, she would like to see ideologies, whether political or religious, removed from the equation of how we deal with the disease, in particular the predominantly Christian dogma that it is immoral to promote condoms.

"I guess my moral framework has become a lot more big picture' since I started out in this field," she says. "I now feel that it's morally wrong to allow people to die in hundreds of millions because we don't want to recognise that people have sex. We could be doing a lot more about it than we are and we're choosing not to do it under the guise of not wanting to promote immorality. I think that's immoral."

*The Wisdom Of Whores by Elizabeth Pisani is published this week by Granta, £17.99