An expert in mass psychogenic illness has told the ABC's Signal daily news podcast that a string of mysterious illnesses among US diplomats is caused by a form of "mass hysteria", rather than by a previously unknown type of sonic weaponry.

Key points: The US State Department is investigating whether US diplomats are being directly targeted by "acoustic attacks"

The US State Department is investigating whether US diplomats are being directly targeted by "acoustic attacks" Dr Bartholomew says it is "physically impossible" for sonic weapons to cause concussions

Dr Bartholomew says it is "physically impossible" for sonic weapons to cause concussions Others suggest diplomats may be suffering from previously unknown condition

There have now been 26 Americans confirmed by the US State Department to have been affected by the mysterious condition, with cases reported in Cuba, China, Uzbekistan and — in the past few days — Singapore.

The Singapore case has since been described as a false alarm, but US officials remain on high alert; the State Department has now implemented a policy of immediately investigating any reports from staff of suspicious sounds or unexplained symptoms out of "an abundance of caution".

Since the phenomenon was first documented in Cuba in 2016, US diplomats have reported symptoms including headaches, nausea, vertigo and temporary hearing loss, with tests of some suggesting physical changes to the brain's white-matter tracts, which allow parts of the brain to communicate.

The US State Department confirmed last year it was investigating the possibility that US diplomats were being deliberately targeted in "acoustic" attacks, but New Zealand-based medical sociologist Robert Bartholomew — who specialises in mass psychogenic illness — has told The Signal podcast he doubts nefarious foreign weaponry is involved.

"I am convinced that what we're dealing with is a case of mass hysteria and mass suggestion," he said.

"Sonic weapons cannot cause concussions, or white-matter tract changes — it is physically impossible.

"For me this whole case can be summed up in a single sentence: 'when you hear the sound of hoofbeats in the night, first think 'horses', not 'zebras'.

"These people at the State Department are thinking, "unicorns" — they've gone for the most exotic hypothesis, and one that's scientifically impossible [because] sonic weapons cannot cause these conditions."

Several diplomats have been evacuated from the US consulate in Guangzhou with health concerns. ( AP: Kelvin Chan )

'Concussion without concussion': experts hypothesise new illness

Twenty-one of the US diplomats evacuated from Cuba in 2016 were the subject of testing conducted by the State Department and by doctors from the University of Pennsylvania.

Those doctors published an article reviewing the cases in the prestigious Journal of the American Medical Association (JAMA) in February, suggesting the symptoms resembled "concussion without concussion" or a mild brain injury without any obvious signs of trauma.

Speaking on a podcast produced by the JAMA after publication, one of the co-authors, neurosurgery professor Douglas Smith, said the symptoms could indicate a new condition altogether, and suggested there was a need for further analysis.

"It really looks like concussion without the history of head trauma, and a thing we are pretty certain of is that it was not the sound itself that caused the injury," Dr Smith said.

But Dr Bartholomew is sceptical of that finding, and is one of several experts criticising the way the study was conducted and described.

He claims the conclusion around 'concussion-like symptoms' should be treated with suspicion, and thinks the suggestion that the diplomats might be suffering from a previously unknown condition is wrong.

"White-matter tract changes are common in everything from dementia, to depression, to normal ageing," he told The Signal.

"The 'concussion-like symptoms' reported by the authors in the recent JAMA study, when you look at the actual data, there were no structural changes to the brain.

"These were based on scans, and the scans are open to interpretation."

Cases 'may have been caused by surveillance device'

Sorry, this video has expired Mike Pompeo says attack in China 'entirely consistent' with Cuba attack

Another theory being offered to explain the strange cases is that the US diplomats and others are being affected by a sound that was a by-product of a bugging device installed inside US diplomatic missions by a foreign power.

Speaking to The Signal, Foreign Policy magazine executive editor for news Sharon Weinberger, who has written a book on weapons technology including sonic weaponry, said the cases reminded her of symptoms reported by American diplomats based in the US embassy in Moscow during the Cold War.

In those cases, she says, it later emerged the symptoms were not caused by sonic weapons or by mass hysteria, although both explanations were offered at the time.

Instead, the US has since concluded the cases were caused by microwave radiation from bugging devices installed inside the embassy to snoop on American diplomats.

"In Moscow, it turned out that the Soviets were irradiating the embassy because they were activating listening bugs in the wall," she said.

"That is something that you would expect — you would expect foreign countries to surveil American diplomats."

Asked if she thought a similar phenomenon might be at play in the cases in China and Cuba, Ms Weinberger told The Signal she thought it the most likely scenario.

"I don't think mass hysteria — I do not think that State Department diplomats in China and Cuba are suffering from mass hysteria," she said.

"Look, I've lost many bets in my life by prognosticating, but if I were forced today to place my money on something, I would guess that some of the cases were caused by some sort of surveillance device."

But Dr Bartholomew is undeterred in his hysteria theory.

"The people who are most susceptible to mass hysteria are those that think they are not susceptible," he said.

"I think what we're seeing is a combination of ever present background noises — crickets, cicadas — and social paranoia, and tinnitus — there's a lot of people who have tinnitus.

"It is the placebo effect, in reverse."