Nothing is ever just an accident. Could the European Union be a front to restore the Merovingian dynasty and the bloodline of Jesus? Did George W. Bush stand aside and allow 9/11 to happen - or even, as the more extreme "truthers" argue, make it happen? Did a dastardly Duke of Edinburgh spearhead a plot to kill the lovely Diana, Princess of Wales? And were the Apollo moon landings all faked by Nasa - as 6 per cent of the US population believe?

We live, it is often argued, in an era of conspiracy theories - many of them as daft as those just related. "Sometimes it appears as if Western societies have regressed," suggests Frank Furedi, professor of sociology at the University of Kent, "and we are now adopting a medieval attitude towards calamitous acts." Whenever something goes wrong, "a simplistic, conspiratorial world view emerges to blame small cliques of evil people".

Sarah Churchwell, senior lecturer in American literature and culture at the University of East Anglia, agrees. Conspiracy theories provide "a belief system in an age of uncertainty and unfiltered information that destabilises knowledge with so many 'facts' and possible interpretations. Conspiracies attempt to make order out of chaos."

Churchwell is the author of The Many Lives of Marilyn Monroe (2004), which examines - and explodes - the myths that have grown up around the film star's life and death. "The conspiracies are completely ongoing; they build and build and keep on proliferating. There's never a story to which Marilyn can't be attached," she says.

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There are also a number of prominent revisionist claims about significant issues - that climate change is a fiction, that HIV has no connection with Aids, that the scale of the Holocaust has been greatly exaggerated - that are almost invariably linked to the suggestion that mainstream scholarship is inherently suspect and corrupted by political or material interests. Self-styled maverick researchers portray themselves as victims, sidelined or silenced by the powers that be. Even the process of peer review can be presented as part of a conspiracy to shut out critical voices, to police knowledge and to protect received wisdom.

Perhaps it hardly matters if a Dan Brown fan laps up every word of The Da Vinci Code and then proclaims on television that it had made him realise there had long been a cover-up of the fact that Mary Magdalen could have been one of the Spice Girls. Yet many conspiracy theories are libellous, dangerous and ultimately corrosive of serious intellectual debate.

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Fortunately, help is at hand - from the academy. Who better than a sober historian to pour the cold water of reason and evidence on overheated conspiracy claims? If we really want to know whether there's been a plot to conceal who discovered America, or if the Ancient Greeks stole their best ideas from Africa, there are people in universities who can tell us.

Kathryn Olmsted, professor of history at the University of California, Davis, recently published Real Enemies: Conspiracy Theories and American Democracy, World War I to 9/11. She believes that if conspiracy theories are the problem, most academics are part of the solution. "Almost by definition, conspiracy theories are simple ways of telling complicated stories, and academics are averse to oversimplifications. Historians have been especially active in knocking down conspiracy theories about the distant or relatively distant past" - such as the idea that President Franklin D. Roosevelt knew about or provoked the Japanese attack on Pearl Harbor in 1941.

So far, so good. Academics undoubtedly can and do play such a salutary role. But is this the whole story? Are academics always on the side of the angels, rallying to the cause of truth - or can they sometimes be part of the problem?

Definitely the latter, says journalist David Aaronovitch, who has recently published Voodoo Histories, a book on conspiracy theories, as part of what he sees as a wider "war against stupidity, designed to establish evidence-based foundations for thinking". He also hopes to help people "distinguish between the scholarly and the slapdash, the committed researcher and the careless loudmouth, the scrupulous and the demagogic".

When it comes to gullibility, Aaronovitch suggests, neither the academy nor his own profession should feel too complacent. "We are used to seeing gross prejudices as the product of peasant credulity, lumpen ignorance or provincial small-mindedness. People like us, this implies, would not be fooled." But it just ain't so.

We may not be surprised that in Nazi Germany, academics, journalists and other educated professionals queued up to heap praise on The Protocols of the Elders of Zion, the ridiculous but lethal forgery said to demonstrate the existence of a global Jewish conspiracy. But there has also been an eloquent and highly visible, though obviously small, minority of British and American academics who have played a major role in promoting conspiracies about Roosevelt, President John F. Kennedy and the events of 9/11, not to mention the idea that Jesus made a botched attempt to fake his own crucifixion.

When it comes to spinning conspiracy theories, Aaronovitch claims provocatively: "Academics, students and journalists are the most innovative sector. You need a pretension to knowledge without real knowledge. What better than academics talking outside their field?"

People working in disciplines such as theology and peace studies are said to be particularly susceptible, whereas "historians have to have a sense of how things don't happen - which makes conspiratorial thinking implausible. Few experts on structural physics and engineering have signed up for 9/11 theories," he notes, referring to claims such as the one that the World Trade Centre's Tower Seven was destroyed by a controlled explosion.

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Ronald Fritze, professor of history at Athens State University, Alabama, who recently published Invented Knowledge: False History, Fake Science and Pseudo-religions, is also concerned about academics who stray off their home turf.

"Some are prominent Holocaust deniers," he says, "but they tend to have posts in engineering and business faculties that have nothing to do with their ideas about the Holocaust. Barry Fell (1917-94) believed that Europeans reached North America during the megalithic era. He is put forward as a Harvard professor to bolster his credibility, but the problem is that his expertise was in marine biology, not archaeology, prehistory or ancient Celtic languages."

In other areas of conspiracy theory, however, it is only experts who are likely to be listened to. Any pub bore can acquire the basic knowledge of ballistics and grassy knolls needed to "prove" that Lee Harvey Oswald could not have shot Kennedy. But there is no such thing as robust common sense about retroviruses. Here too, alas, it is often argued that the academic record is far from perfect.

Seth Kalichman is author of the recently published Denying Aids: Conspiracy Theories, Pseudoscience, and Human Tragedy. He is also professor of psychology at the University of Connecticut, which makes him, as he notes in an amusing introductory declaimer, "an employee of The State". He has never, he assures us, "taken financial support from any pharmaceutical company, although he has accepted pens and key chains from Pfizer sales reps at conventions".

Despite his jokey tone, the stakes could hardly be higher. Kalichman carries out Aids research in South Africa, where denialism - the claims that "the HIV = Aids myth is the product of a government conspiracy in cahoots with a multibillion-dollar pharmaceutical scam" and that antiretroviral medicines are toxic - has had a huge and disastrous impact on government policy. A prominent biologist, Kalichman argues, must take a significant share of the blame for this disaster.

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"There are only a handful of academics who are involved in spreading conspiracy theories," says Kalichman, "but they are quite destructive. I am not one for placing any limits on free speech or academic freedom, but these characters help to make the case.

"Those who are tenured are unstoppable. Tenure committees have to be more critical than ever in their scrutinising of journals, because pseudoscience now has outlets that would typically appear legitimate.

"The most destructive people linked to conspiracy theories and denialism are those with academic appointments - and those who can manipulate their backgrounds to appear as if they have had academic appointments."

Because many conspiracy theories are pernicious as well as absurd, their promotion by fringe groups of academics claiming the prestige of their universities is a source of concern. But what can others do to fight back?

It isn't always easy. Genuine scholars can be at a disadvantage in formats such as television's battle of the soundbites. Conspiracy theorists can put forward a simple, sensationalist and emotionally satisfying thesis that can be demolished only painstakingly, brick by brick. Unless academics are exceptionally skilled and quick-witted debaters, they can easily come across as pedantic killjoys as they labour to unpick conspiratorial ideas while also countering attempts to impugn their motives.

Such hazards, Kalichman suggests, explain why "scientists and other genuine academics are reluctant to enter into a 'debate' with conspiracy theorists and denialists. It is easy to get caught in a trap. The denialists will often walk away looking like righteous heroes standing up against the corruption of government, industry and the Establishment."

Fritze takes a similar line. "Academics tend to avoid controversies concerning pseudohistory and pseudoscience because they can get roughed up and dragged into quagmires of circular debate."

Churchwell has no doubt where the more conspiratorial biographers of Marilyn go wrong. "The more you read about her, the more you realise how little we know. There are all sorts of legitimate questions that we don't know the answers to. But there are some things we do know. I can unpick the lies. I can demonstrate falsity.

"You need to think sceptically about the role of evidence. Lots of stuff one can point to has its origins in gossip or hearsay but is taken seriously because it's 'in the record'. A mix of commerce and laziness drives most of the theories."

But although Churchwell has taken part in myth-busting documentaries and hopes her book "has convinced some who were willing to be convinced", she also knows that "getting involved in public debates would be an exercise in futility, because conspiracy theorists are unscrupulous and abusive and come with their minds made up".

Fortunately, however, conspiracy theorists don't always get things their own way. Popular Mechanics magazine and a number of websites have systematically addressed the "suspicious anomalies" noted by the self-styled "9/11 truthers". Mohamed Al Fayed's determination to have his day in court led to a public discrediting of his claims about Prince Philip's role in Diana's death.

Something similar occurred when David Irving unsuccessfully sued the academic Deborah Lipstadt and her publisher Penguin Books for libel in a UK court after she described him as "one of the most dangerous spokespersons for Holocaust denial". This gave Richard Evans, now Regius professor of modern history at the University of Cambridge, the opportunity to submit a 700-page report that spelt out in detail the "knotted web of distortions, suppressions and manipulations ... the sheer depth of duplicity encountered in Irving's treatment of the historical sources".

Under cross-examination, Evans proved highly effective in dodging Irving's debating tricks and doggedly answering him point by minute factual point.

There was a moment when Irving asked Evans to clarify a comment in his report.

"Would you like to point me to the page?" Evans responded. "You see, I have a problem, Mr Irving, which is that, having been through your work, I cannot really accept your version of any document, including passages in my own report, without actually having it in front of me."

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Perhaps a similarly robust approach is the best way of dealing with conspiracy theorists trying to prove that black is white.