Scientists must engage more with the public or the HIV/Aids deniers will gain credibility, argues Tara Smith.

There is no Aids epidemic in Africa. HIV doesn't exist. People are dying because pharmaceutical companies are poisoning them with toxic antiretroviral drugs, not because they're infected with a deadly virus. And scientists investigating HIV are getting rich by propagating the theory of its existence, which they know is a fabrication.

Ridiculous, right? Wasn't this settled 20 years ago?

That HIV causes Aids is no longer a scientific controversy. But for groups that deny that the virus leads to the disease, no amount of evidence has been enough to convince them. And because these groups seek to spread their distrust of the HIV/ Aids paradigm to others, academics must remain vigilant. It is up to us to explain the science to the public and be aware of the methods used by denial groups, because the consequences of HIV denial can be lethal.

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For any scientific theory there are initially gaps in understanding. These gaps allow for - and encourage - scepticism of the proposed hypothesis and active investigation to fill these gaps. This is a healthy and necessary part of science. As our knowledge base increases, some hypotheses will become better supported by data and others will fall by the wayside as the tide of evidence turns against them, eventually leading scientists to reach a consensus view.

With HIV deniers, accumulation of evidence is never enough. New research causes them to "push back the goalposts", to always demand more evidence than can be provided. When it is provided, deniers push the goalposts back again, or refuse to accept that the evidence meets their demand.

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One way to brush off novel evidence is to attempt to discredit the scientists carrying out the studies. This is another hallmark of the HIV denial groups. While they hold up some "dissident" scientists as heroes to their cause, they accuse those who accept mainstream ideas of "selling out" to the pharmaceutical industry, despite the fact that many researchers receive no funding from any such company. For those who receive no industry support, the deniers argue that it is still in the scientist's best interest to maintain the status quo. This allows deniers to summarily dismiss scientists who support HIV causation of Aids as either greedy, since supporting the mainstream paradigm brings these researchers financial gain via salary or research funding, or as cowards and "sheep" who are afraid or unwilling to speak out against the prominent theory (which deniers argue many scientists "know" is incorrect, but support nevertheless).

In the end, Aids science becomes a grand conspiracy, controlled by the pharmaceutical industry with scientists and doctors as willing pawns. Therefore, vast swaths of the literature can be brushed away by deniers, leaving only their fellow "dissidents" as the scientists with enough courage and integrity to speak against "Aids Inc".

These strategies are not unique to HIV denial. Other groups, such as those who deny evolution or climate change, employ similar strategies: moving goalposts, assigning ulterior motives to mainstream scientists to discredit them, cherry picking evidence. They also all commonly spend the majority of their efforts critiquing the mainstream theory, wrongly assuming that if the dominant paradigm is overturned their own unproven ideas will fill the void.

Though based mainly in the US and Canada, prominent HIV deniers have worldwide influence via the internet and beyond. In 2000, South African President Thabo Mbeki invited self-styled HIV "dissidents" to a conference discussing the aetiology of Aids, expressing doubt that the virus was the causative agent. Mr Mbeki has since backed off these statements, but his presidency remains embroiled in battles over treatment and management of the disease - battles that Aids activists argue are costing lives.

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Scientists, rightly, scoff at this denial because the ideas put forth by HIV denial groups were long ago discredited in the scientific literature. However, ignoring these fringe groups only allows their misinformation to spread unchecked through cyberspace, where many readers will not have the scientific background necessary to analyse the deniers' claims. Instead, they may not only accept but also propagate these fringe ideas, potentially putting themselves and others at risk. Scientists, medical doctors and health educators need to be aware of these groups and the strategies they use and, when possible, engage the public more actively in an understanding of science and the scientific method.

Tara Smith is assistant professor in the department of epidemiology at the University of Iowa.