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No biomedical cause was found in either and experts believe they were the result of mass sociogenic illness, Prof Wessely said.

Prof Simon Wessely, a psychiatrist and epidemiologist at King's College London, said several "key epidemiological factors" led him to believe these were not a chain of poisonings, but were instead a case of "mass sociogenic illness" - in which symptoms spread among a group with no obvious biomedical cause.

In cases of mass sociogenic illness - which used to be described as "mass hysteria" - the symptoms experienced are real, but they are caused by anxiety, not toxic poisoning, Prof Wessely said.

Could it be that a great swathe of America is really undergoing what doctors refer to as "mass sociogenic illness" when it comes to gluten?