Most doctors, nurses, health educationists and others on the AIDS front line concur. They are given to understand by the general scientific community that even to doubt HIV shows mental and moral deficiency. Scientists, too, have to be careful not to rock the HIV boat, which carries jobs, reputations and huge research funds.

Despite this pressure, a large and growing network of highly-qualified "dissidents" has become established worldwide over the past two years. They not only challenge the HIV hypothesis, but have "come out" publicly about their concerns. More than 450 have put their names to a letter demanding a reappraisal of the conventional view, arguing that the HIV hypothesis is at best unproven, at worst discredited.

The group contains more than 70 PhDs, scores of medical doctors and numerous other health workers and scientists, along with AIDS patients, activists and others who have been working for years with those most affected by the epidemic. Most of the names are American-based, but overall the list spans 23 countries.

It is the tip of an iceberg of dissent. The group's newsletter has a mailing list of more than 2,000. A debate over AIDS causation is breaking out around the world, reflected in a wave of conferences highlighting alternative ways of thinking about the disease. The latest of these takes place in Bologna, Italy, this month.

The challenge has its origins in theoretical considerations first set out in detail by Professor Peter Duesberg, an eminent American virologist. But it has drawn strength from the failure of AIDS researchers, after 10 years' work on HIV, to show how the virus could be doing the damage attributed to it. Gallo claimed to have shown it made its victims vulnerable to infections by directly killing cells of their immune system. It is now known he was wrong.

A further blow to orthodoxy has been the failure to show any lasting benefit from expensive but highly toxic and potentially lethal anti-HIV drugs.

A third concern is that the link between HIV and AIDS is much less close than had been thought. HIV is not the "death sentence" many were given to understand. The collapse of doomsday predictions about HIV's spread (3m UK HIV-positives by 1990, according to a 1985 forecast; actual 10-year cumulative total to date: 21,000) has become apparent in most countries with careful testing programmes.

The HIV test itself is discredited. There has been no reasoned response to scientists who argue it was never properly validated, and that false positives are the rule not the exception. A crisis looms, probably the biggest faced by the modern scientific community.

All the more reprehensible, then, in a discipline dedicated to openness, that most doctors and scientists are unfamiliar with the dissidents' detailed arguments because leading scientific and medical journals refuse to publish them.

Despite the issue's importance, journal editors and advisers seem to reason, like Gallo, that since "everyone" agrees HIV is the cause of AIDS, anyone seriously proposing otherwise is eccentric and illogical. Dissidents are dangerous, it is argued, because if people doubt HIV's role in AIDS, they might not heed warnings about the need to change sexual habits.

John Maddox, editor of Nature, the science journal, feels so strongly on the issue that he has been trying to dissuade this newspaper from reporting evidence that the virus is probably not the cause of AIDS, and that AIDS may not be an epidemic putting everyone at risk.

Other publications, lay and professional, follow a similar line. New Scientist magazine, under the headline "HIV: beyond reasonable doubt", declared that despite "a mountain of scientific evidence" showing that HIV does cause AIDS, "a small number of journalists and dissident scientists continue to question the link". That "mountain" of evidence never existed; the HIV consensus is crumbling. Maddox admits that the failure to find a mechanism for how HIV causes the disease has been "a profound disappointment to the research community", and that the only evidence implicating the virus an epidemiological association between testing HIV-positive and being diagnosed as having AIDS "necessarily seems circumstantial".

Signatories of the reappraisal letter are united in wanting a change in direction; they differ in the extent to which they reject the HIV theory.

Some, like Dr Charles Thomas, a molecular biologist and former Harvard professor of biochemistry, say it is complete nonsense. "The HIV-causes-AIDS dogma represents the grandest and perhaps the most morally destructive fraud that has ever been perpetrated on the young men and women of the Western world," he says.

Equally outspoken is Phillip Johnson, senior professor of law, University of California at Berkeley, and a former visiting professor at University College London.

"One does not need to be a scientific specialist to recognise a botched research job and a scientific establishment that is distorting the facts to promote an ideology and maximise its funding," he says. "That establishment continues to doctor statistics and misrepresent the situation to keep the public convinced that a major viral pandemic is under way when the facts are otherwise."

Others, like Dr Lawrence Bradford, a biology professor in Atchison, Kansas, and Dr Roger Cunningham, a microbiologist and director of the centre for immunology at the State University of New York at Buffalo, think the virus could be one factor among many, but maintain an unbiased reassessment is urgently needed.

"Unfortunately," Cunningham says, "an AIDS 'establishment' seems to have formed that intends to discourage challenges to the dogma on one side and often insists on following discredited ideas on the other."

Bradford lists drug use, multiple infections, exposure to blood products, rectal exposure to semen, and autoimmune phenomena in which the immune system becomes so confused that it starts to self-destruct as among interacting events that may lead up to AIDS.

Many other signatories hold a similar view. Professor Arthur Gottlieb, head of microbiology and immunology at Tulane University medical school, New Orleans, believes HIV can disrupt the immune system, but lists several co-factors, including malnutrition, as important.

Dr Steven Jonas, professor of preventive medicine, State University of New York at Stony Brook, says evidence is now "rapidly accumulating" that the original theory of HIV as the sole cause of AIDS is not correct. He thinks it plays a part in most cases, but "by itself it is not sufficient to cause the disease".

Dr Alfred Hassig, former professor of immunology at the University of Bern and director of a Swiss blood transfusion laboratory, says multiple stresses on the immune system provoke an acute reaction, allowing latent microbes, including HIV, to proliferate. He believes dietary measures can reverse this process, and urges that "the sentences of death now accompanying the medical diagnosis of AIDS should be abolished".

Dr Gordon Stewart, professor emeritus of public health, Glasgow University, and a former World Health Organisation AIDS adviser, who links AIDS in Western countries to behaviours carrying high risks of genital and other infections, points out that the implication of such alternative views is that existing efforts to fight AIDS are "very wasteful of effort and expenditure".

Most of the signatories, such as Dr Henk Loman, professor of biophysical chemistry at the Free University in Amsterdam, deplore the neglect of non-HIV lines of research. "There are many people with AIDS but without HIV, and a great many people with HIV but without AIDS," says Loman. "These two facts mean that HIV AIDS is much too simple. Plausible, alternative, testable causes of impairment of the immune system which may ultimately lead to AIDS should become part of regular AIDS research."

Many of the scientists believe the fight against AIDS was derailed by a flaw in reasoning over HIV in which "the hypothesis itself got incorporated in the definition of AIDS," as Dr Kary Mullis, winner of last year's Nobel prize for chemistry, puts it. When people fall sick and HIV is present or thought to be present, it is called AIDS; when HIV is not present, it is called something else.

Mullis, who invented a genetic test used worldwide by AIDS researchers, says that "the HIV theory, the way it is being applied, is unfalsifiable and therefore useless as a medical hypothesis". He believes AIDS arose through "an enormous level of exposure to human viruses and bacteria".

Robert Maver, consulting actuary to healthcare and insurance industries, and former vice-president and director of research with the giant Mutual Benefit Life Insurance Co, agrees that incorporating HIV into the definition of AIDS "has created a high correlation that is tautologically contrived".

Harry Rubin, professor of molecular and cell biology, University of California at Berkeley, declares himself "an HIV agnostic. It is not proven that AIDS is caused by HIV infection, nor is it proven that it plays no role whatever in the syndrome."

An Italian signatory, Dr Fabio Franchi, a specialist in preventive medicine and infectious diseases in Trieste, declares: "I am not agnostic; I am well convinced, above all by the arguments of Professor Peter Duesberg." Duesberg, professor of molecular biology at the University of California at Berkeley, and a founder of the reappraisal group, says HIV is harmless, and believes long-term misuse of both recreational and medical drugs is the prime cause of AIDS.

Equally sure of HIV's innocence is Dr Bernard Forscher, formerly managing editor of the Proceedings of the National Academy of Sciences. "The HIV hypothesis ranks with the 'bad air' theory for malaria and the 'bacterial infection' theory for beriberi and pellagra (caused by nutritional deficiencies)," he says. "It is a hoax that became a scam."

Paul Rabinow, professor of anthropology, University of California at Berkeley, who interviewed Duesberg after becoming "intrigued by the highly emotional reactions" to his ideas, found there were no convincing answers to his questions. "In particular I have pursued the situation of haemophiliacs and HIV because it was the arena least charged with moralism. Systematic attempts to be allowed to look at the data or to find rigorous controlled studies have failed."

Another Berkeley scientist, Dr Richard Strohman, emeritus professor of molecular and cell biology, is concerned that "while it becomes increasingly clear that factors other than HIV must be involved in AIDS, the major research effort continues to focus on the virus".

Harvey Bialy, research editor of the New York-based journal BioTechnology, who has a background in molecular biology and tropical disease study in West Africa, insists that "AIDS" in Africa can mostly be attributed to economic decline, decline in health care and development of drug-resistant infections. "All these things can explain exactly what is going on, to much greater good for the public health than saying the diseases are being made worse by HIV," he says.

Several signatories say that apart from misdirecting energies, the HIV theory has caused great psychological damage. Paul Lineback, counselling psychologist, Eastern Oregon State College, says: "Protecting and promoting the unproven HIV hypothesis as fact is inducing unnecessary stress, probably emotional harm, and maybe even psychological murder."

Michael Ellner, president of Heal (Health Education AIDS Liaison), based in New York City, a medical hypnotherapist who has worked with thousands of people with AIDS-related fears and conditions, says: "I have seen the constant terror, and programming to get sick and die, that people at risk for developing AIDS face. I am certain that the hypothesis that long-term drug use is a primary cause of what is now called AIDS is far more likely to prove true than the failed notion that AIDS is caused by a germ."

According to David Mertz, a philosopher of science at the University of Massachusetts, Amherst, HIV is an "entity of convenience" that met the needs of powerful groups: researchers competing for personal and national prestige after the failure of the United States government's "war on cancer"; gay civil rights campaigners "who wanted to remake AIDS as an 'equal opportunity killer'," and the right wing, "who wanted an agent to concretise the 'wrath of God' that they fantasised as visited upon gays".

If Mertz is right, will the illusion ever end?

Professor Hiram Caton, head of the school of applied ethics at Griffith University, Brisbane, Australia, believes it will. The orthodox view will collapse, he says, "because it flunks the practical tests. No vaccine will be forthcoming. No effective treatment will appear. The hype will exhaust its credibility.

"Scientists will then have to come to terms with the awful fact that the AIDS epidemic was a mirage manufactured by scientists who believed that integrity could be maintained amidst the diverting influences of big money, prestige and politics." *