A middle-aged man walks into an East London café and apologises for being late. With his clipped hair and bus-driver's uniform of thick overcoat, shirt, and branded tie, he looks like any other public service employee. But soon he delivers a speech of startling ferocity against the medical establishment.

Mike explains that he runs a London-based health website on which he posts articles and links to information that questions whether HIV causes Aids, disputes the existence of HIV, and denies the fact that unprotected sex helps to spread it. He offers support for those who, he says, are "negotiating with medical authorities over taking a different approach to dealing with their circumstances." He claims to get thousands of hits on his site and has helped advise several people who have been diagnosed with HIV and are launching legal action against their local health authorities, in the belief that they have been unfairly treated by the doctors who are trying to help them.

Mike is an Aids denialist. He shares the view of a global network of academics and campaigners that follow the proclamations of Peter Duesberg, a cell biologist at the University of California, Berkeley, who believes HIV does not cause Aids. And, alarmingly, 2009 has been a good year for the denialist community.

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In the first week of November, a record number of Aids denialists from 28 countries, including Britain, attended the Rethinking Aids conference in Oakland, California. One of the main draws of the conference was a screening of a controversial new documentary by Canadian-born director Brent Leung, House of Numbers, which gives a platform to denialist theories.

Over the last two months it has been screened at the Cambridge and Raindance Film Festivals – decisions that provoked a storm of criticism online. The Spectator was forced to cancel a debate and screening of the film on 28 October after some of the participating speakers pulled out. And yet despite widespread outrage, the film has undoubtedly encouraged those who espouse denialist theories in the UK.

So who are the Aids deniers and what do they believe? According to Seth Kalichman, a psychologist at the University of Connecticut, whose exposé of the movement, Denying Aids, was published in March, denialists anywhere in the world generally share several common beliefs. They say that the "myth" that HIV causes Aids is the product of conspiracies between governments and the pharmaceutical industry; that antiretroviral medication is toxic; and that one day the orthodox medical theories on HIV will crumble.

So far, so typically crackpot. But the movement has gained some damaging traction – and the propagation of denialist theories can have deadly repercussions. Aids charities warn that reading material which argues that HIV does not cause Aids can dissuade potential sufferers from getting tested for HIV, and even lead HIV-infected people to ignore HIV-positive results and cause them to reject antiretroviral therapies.

"Denying the link between HIV and Aids is scientific illiteracy," says Yusef Azad, director of policy and campaigns at the National Aids Trust, Britain's leading HIV/Aids charity. "But worse than that, it is profoundly dangerous and has caused countless unnecessary deaths. Just because something is on the internet does not mean it is even remotely true. More than two decades of peer-reviewed scientific research demonstrates in some detail how HIV attacks the immune system and causes Aids if left untreated."

***

Today is World Aids Day, an annual event designed to raise awareness of the problems facing the 33.2 million people around the world who live with the disease. The initiative's British website outlines one of its aims: to "present true, sometimes surprising, accounts of how HIV affects people in the UK, and to dispel myths and misinformation."

The Joint United Nations programme on HIV/Aids and the World Health Organisation declared last week that the HIV/Aids pandemic was on a downward trend for the first time; in part due to the use of effective anti-retroviral treatment. With the evidence on orthodox approaches to HIV being so overwhelmingly convincing, why do some insist on spreading health advice that could put other people's lives at risk? And who believes them? The answer lies in some scientifically discredited research that was publicly aired almost 30 years ago, but still drives a disparate network of supporters.

According to international Aids charity Avert, Aids science began in the 1980s, when some scientists linked an outbreak in opportunistic infections – those diseases which take advantage of a lowered immune system – with people's lifestyle choices. In December 1981 a research paper was published in The New England Journal of Medicine which reported the first cases of PCP (pneumocystis carinii pneumonia, a rare condition, later discovered to be Aids-defining) among intravenous drug users.

In February 1982 a group of scientists published a paper in The Lancet that put the opportunistic infections down to the use of the stimulant amyl nitrate, or "poppers" among the gay community (a letter in The Lancet in 1981 went as far as to call the problem "gay compromise syndrome"). Soon, however, research indicated that the disease occurred in other population groups. In July 1982, scientists published research on PCP found in people with haemophilia. Fast-forward to 23 April 1984, when the US health and human services secretary, Margaret Heckler, announced that US biomedical researcher Robert Gallo of the National Cancer Institute had isolated the virus which caused Aids. But not all scientists agreed this virus was to blame. It subsequently turned out that Gallo's virus, which he then called HTLV-III, was in fact the same virus discovered a year earlier by Luc Montagnier of the Pasteur Institute in Paris, which he had called LAV.

Peter Duesberg, a Californian cancer researcher who had mapped the genetic make-up of a virus similar to HIV, disputed the predominant theory that a virus was causing Aids. He believed a single virus could not disable someone's immune system and in 1987 wrote a paper for the medical journal Cancer Research that clung to the discredited idea that Aids was caused by long-term consumption of recreational and antiretroviral drugs. He claimed HIV was a "passenger" virus, a harmless type of virus sometimes found in diseased tissue. While at the time, there were others who disputed the orthodox HIV hypothesis, most have since changed their minds as the evidence became overwhelming. Indeed, the sheer quantity of research supporting the HIV/Aids hypothesis is impressive. To cite just one piece of evidence: in 1993, the US Centers for Disease Control and Prevention in Atlanta, Georgia reviewed 230,179 cases of people with an "Aids-like illness". Only 47 people tested HIV-negative, or less than 0.025 per cent.

The Independent found itself at the forefront of the fightback against denialist theories. In May 1992, three Nobel laureates wrote to Steve Connor, this newspaper's science editor, praising his reporting on the issue. Signed by former president of the Royal Society Aaron Klug, the biochemist Cesar Milstein and the molecular biologist Max Perutz, the letter states: "There is no question in our minds – as in the great majority of scientists who have acquainted themselves with the facts – that HIV is the cause of Aids."

But Duesberg – now an outcast to mainstream medical science – continues to push his ideas, winning credence among those who are, for their own reasons, unwilling to accept the truth. Prominent present-day denialists include Henry Bauer, a retired chemistry and life sciences professor at Virginia State University and Eleni Papadopulos-Eleopulos, a medical biophysicist working at Royal Perth Hospital, at the University of Western Australia. All three have a strong following online. Duesberg's website gets 15,000 hits a month, and has links to articles like "Duesberg Defends Challenges to the Existence of HIV".

The influence of such literature should not be underestimated. "Internet postings suggest thousands of people at least question the science behind HIV as the cause of Aids," warns Seth Kalichman. "In 2007 there were 'Aids dissident' science conferences held in Paris and Berlin. An online Aids dissident encyclopaedia-style website, Aids Wiki, boasts over 70,000 visits. The proliferation of denialist writings through multiple media outlets does more than distract Aids scientists; it undermines countless efforts to save lives."

***

One only needs to look to South Africa to see the lethal consequences of denialism. The former South African president, Thabo Mbeki, discovered the movement and in 2000 he assembled a "Presidential Aids Advisory Panel" of 30 people, half of whom were denialists, including Duesberg and Papadopulos- Eleopulos. In 2000, Mbeki's opening remarks to the panel included the words: "There is an approach that asks why this President of South Africa is trying to give legitimacy to discredited scientists, because, after all, all the questions of science concerning this matter had been resolved by the year 1984. I don't know of any science that gets resolved in that manner with a cut-off year beyond which science does not develop any further." Partly on the advice of denialists, Mbeki's administration did everything in its power to resist the use of ART, the antiretroviral therapy that stops HIV "replicating" – essentially producing copies of itself and spreading throughout someone's body – and prevents Aids. Last year, a study by researchers at Harvard University published in The Journal of Acquired Immune Deficiency Syndromes estimated that if Mbeki's government had provided HIV treatment there would have been a staggering 365,000 fewer premature deaths in South Africa during his leadership.

It may be hard to see how intelligent people are sucked in by the denialists' claims, but Kalichman believes they use a number of tricks, the most common of which is known as "cherry-picking" – isolating individual, out-of-context sentences from scientific papers to illustrate their theories. "They create confusion among people who are not experts in Aids and don't recognise what they are doing for what it is," he says.

"Most people are not well-versed in science and medicine and can be easily confused by misinformation. And the denialist literature is much more user-friendly than true science. Scientists are notoriously bad at communicating with the public. But the thing is, the orthodox scientists are credible, and the denialist ones are not.

"Credibility comes through relying on accepted scientific standards and through peer review – the process that confirms that scientific work is legitimate," Kalichman continues. "People like Duesberg have not had their Aids ideas pass through the filter of peer review. They do not even do research on HIV or Aids. They are good at selecting sentences out of papers to support their arguments while ignoring all that does not fit in their preconceived notions. When it comes to Aids they do not respect the rules of science."

If you look in the right places, of course, denialist rhetoric is easy to debunk. For instance, according to New Scientist, denialists often claim that HIV has never met "Koch's postulates" – a list of conditions drawn up by the 19th-century German scientist Robert Koch that need to be met in order to prove that a particular infectious agent causes a disease. "HIV does, however, meet Koch's postulates as long as they are not applied in a ridiculously stringent way," wrote Jonny Steinberg in the 22 June issue of the magazine.

The first postulate states that the infectious agent must be found in every person with the disease – this is strongly indicated by the US Centers for Disease Control and Prevention study cited above. Another postulate says this agent must cause the disease if given to a healthy person. In three separate incidents, US laboratory workers accidentally exposed to purified HIV tested positive for that specific strain and later developed Aids.

So who believes the denialist literature? Being "in denial" is often a means for a sufferer to cope with disease diagnosis – and this is a reaction common to other serious diseases. "When you were first diagnosed, you may have trouble believing or accepting the fact you have cancer," says the website of the US National Cancer Institute. "This is called denial. Denial can be helpful because it can give you time to feel hopeful and better about the future. Sometimes, denial is a serious problem. If it lasts too long, it can keep you from getting the treatment you need. It can be a problem when other people deny that you have cancer, even if you have accepted it."

The loved ones of those afflicted by HIV/Aids are also vulnerable to denialist literature. "The same can be true for those close to someone who tests HIV-positive – a friend or lover, a parent, a sibling, or a child," Seth Kalichman writes in Denying Aids. "Those who are in denial are the very people to whom denialists pose the greatest threat. Though denial can, for a time, serve very well as a way of adjusting to the truth, when it goes on too long, it can become maladaptive, keeping us from moving on." One of those most high-profile cases of a sufferer sucked in by denialist literature was that of US Aids activist Christine Maggiore, who was diagnosed with HIV in 1992. Influenced by Duesberg's writings, she waged a long, bitter campaign to contest the fact that the HIV virus causes Aids, and that preventive approaches and antiretrovirals can help thwart the disease's spread and prolong the lives of those who suffer from it.

She breast-fed her daughter, Eliza Jane, who died at the age of three in 2005. The Los Angeles County coroner concluded that the cause of death was Aids-related pneumonia. Maggiore refused to believe it. Her campaign ended last December with her own death from pneumonia, aged 52. No autopsy was conducted at the time, and her supporters still resisted suggestions that Maggiore died from an Aids-related disease. "Why did she remain basically healthy from 1992 until just before her death?" asked David Crowe, president of the Rethinking Aids organisational board, earlier this year.

***

Back in East London, Mike's own story fits Seth Kalichman's archetype. He describes how an ex-boyfriend was diagnosed with HIV when the two of them were still in a relationship, even though Mike discouraged him from taking the HIV test.

After the diagnosis they continued to have unprotected sex, in part fuelled by Mike's belief that he was safe – a belief that had been encouraged by denialist literature (Mike says he has not been diagnosed with HIV). Mike also discouraged his former partner from taking antiretroviral medication, which he says made his former partner sick the one time he did try it. Mike's ex-boyfriend died soon afterwards. Mike says this was, in part, due to an undiagnosed heart defect and potentially the stress caused by his HIV diagnosis.

"I wanted to do something," he says. "I had gone through some intense soul-searching at the time he was diagnosed. I went through everything all over again. I thought, 'Could I have been wrong?' If you genuinely question something, you allow any possible answer. You either dismantle something or else you reinforce your faith. It reinforced my faith [in denialist theories] – because I was really looking where I could be wrong and the orthodox perspective just seemed so weak." So he set up his website.

Another London-based denialist is filmmaker Joan Shenton, who has made 150 programmes for network television. On her website she also lists links to the work of Duesberg and Papadopulos-Eleopulos and claims to get tens of thousands of hits a year. She aims to amass an online database of 120,000 documents comprising what she terms the "changing evidence" surrounding HIV/Aids. "I will tell you why I'm doing it," she says, sitting in her Notting Hill flat. "After my huge illness which was life-threatening from iatrogenic disease [in her case, drug-induced lupus, another autoimmune disease] I barely survived and I continued to work in my profession but I was drawn towards injury from prescribed drugs." Shenton has just returned from the Rethinking Aids conference where she gave a talk entitled "Censorship in the Aids debate – the success of stifling, muzzling and a strategy of silence".

In a paper published in The Journal of Acquired Immune Deficiency Syndromes in 2007 it was shown that as many as one in four people in the US questions Aids orthodoxy. In the UK, the evidence available suggests that the number taking note of denialist theories is peripheral; according to the Health Protection Agency, since the late 1990s, the number of HIV-positive people accessing HIV-related care in the UK has substantially increased. Around 18,000 people were being treated in 1998; this number had more than tripled by 2007.

But does this mean we shouldn't worry about the presence of denialists in the UK? "They are killing people," says Aids expert John Moore, a professor of microbiology at Cornell University. "As a scientist I am offended by those who pervert the profession. I lot of their stuff comes high up the Google rankings and if you have a preconceived idea it can reinforce it."

The internet has undoubtedly been a boon to the denialists. "If you type 'Aids' into a Google Video search you're confronted with a ton of videos, half of which are denialist," says Nicholas Bennett, a fellow in paediatric infectious diseases at the State University of New York who runs an anti-denialist blog. "One of the highest hits you get is a video called Deconstructing the Myth. You get a ton of stuff that is denialist. If people are searching for information, are questioning and looking for a way out of their diagnosis, denialists give them hope. They think they don't have to worry and don't have to start any treatment." Such theories could continue to gain mainstream ground if House of Numbers – which has almost 1,800 fans on Facebook – wins a global distribution deal (it already has a respected publicity agency, Rogers & Cowan, working on its behalf in Los Angeles).

The film lasts about 90 minutes. At the start, Brent Leung, affecting a Michael Moore-style demeanour, is portrayed as an innocent who wants to find out more about Aids. "I was born in 1980, a year before Aids exploded on to the public consciousness," he intones. "I grew up beneath its shadow like a child raised under the threat of the mushroom cloud. You might say I am a member of the first HIV/Aids generation ... This film is an account of my journey through the shifting sands surrounding HIV/Aids." Leung explores many of the topics which so excite denialists – the link between Aids and poverty, the efficiency of HIV testing and even the existence of HIV.

In a review published on 4 September in The New York Times, critic Jeannette Catsoulis describes the film as a "globe-trotting pseudo-investigation that should raise the hackles of anyone with even a glancing knowledge of the basic rules of reasoning". Eighteen of the doctors and scientists interviewed in the film have since issued a statement saying Leung acted "deceitfully and unethically" when recruiting them (Leung says the letter's signatories are anonymous and that they hadn't seen the film when they signed the letter). John Moore, in particular, takes issue over his portrayal in House of Numbers. He says Leung told him he was trying to debunk the claims of denialists in order to secure an interview, but has in fact produced a film that does the opposite.

House of Numbers was shown at the Cambridge Film Festival in September. Bill Thompson, one of the festival trustees, who has since taken personal responsibility for its screening, has described the film as "objectionable, but also an example of a particular genre of deceptive filmmaking that I wanted to show and debunk".

But Elliot Groves, founder of the Raindance Film Festival, which showed the film in October, can be heard on YouTube describing it as "a stunning piece of filmmaking". Also in October, Fraser Nelson, editor of The Spectator, wrote on the magazine's website: "Is it legitimate to discuss the link between HIV and Aids? It's one of these hugely emotive subjects, with a fairly strong and vociferous lobby saying that any open discussion is deplorable and tantamount to Aids denialism. Whenever any debate hits this level, I get deeply suspicious." At the time, Nelson's blog provoked a barrage of criticism.

"Crucially, The Spectator never 'supported' the film," Nelson now says. "What we wanted to do was to screen it, and then have a panel lay their criticisms at the filmmakers. Often in history, the cures to killer diseases come from scientists thinking way outside the box. Aids has killed so many millions that we need to debate it at all possible angles. And if there is a weird, false and scientifically invalid idea then how do we treat it? My answer: that the remedy for bad science is good science. Have a rational discussion."

Is there hope for those sucked into the denialist debate? The National Aids Trust put The Independent in touch with "Daniel", an HIV sufferer in his 30s, who was diagnosed eight years ago. He was initially convinced by denialist literature. "I saw some information written by Christine Maggiore online and it got to the stage where I told my sister I was off to meet with a denialist group in San Francisco," he tells me. "It's difficult to explain why I was so taken in. The thing is, when you are diagnosed, you don't have any symptoms. You get your test back and you don't feel any different ... Some of the denialists do raise questions that at first thought seem plausible. Much of what they quote is from the 1980s, for example. They make these claims over some of the early treatments, which just kept people alive, but things have come on so much since then. There's always half-truths in everything they say."

He says his opinion was altered when was referred for treatment at his local hospital. "I thought to myself, well, if the denialists are telling the truth, then all the doctors and nurses in this hospital are part of some global conspiracy. And it's just absurd to think that. I was a bit nervous about the treatment, but a lot of people are, and that just adds fuel to the fire."

The denialists "are preying on vulnerable people, which is why medics get so worked up about House of Numbers in Britain. It is a serious problem. It puts people off testing and makes doctors' lives harder. Some of these denialists want answers for why they are going to die. But they shouldn't be destroying other people's lives."