In 2016, US workers in Cuba were the targets of a mysterious 'sonic attack' carried out by a new, weaponised sound technology the world had never seen before. Follow-up attacks in China and elsewhere have since confirmed the arrival of this insidious, invisible threat.

But maybe, it could be possible that… nothing ever happened to them.

According to an expert on mass psychogenic illness, the strange symptoms experienced now by 26 Americans in multiple, international episodes could be the result of a contagious, mass delusion – not the hallmarks of an exotic sonic weapon for which there is no actual proof.

"I am convinced that what we're dealing with is a case of mass hysteria and mass suggestion," medical sociologist Robert Bartholomew told ABC News.

Bartholomew has written extensively on mass psychogenic illness – rapid outbreaks within collective groups, showing a variety of symptoms that have no plausible organic basis.

The condition is considered a part of the broader medical and social phenomenon of mass hysteria, which has been observed to cause collective illusions and symptoms across a group of people, not only in recent times, but arguably throughout history.

According to Bartholomew, that's exactly what we're seeing now – even though a comprehensive report released by medical researchers at the University of Pennsylvania in February concluded the victims of the initial Cuba attack suffered "injury to widespread brain networks without an associated history of head trauma".

In other words, the analysis found that the US diplomats – who experienced headaches, vertigo, and hearing loss after hearing strange sounds while stationed in Cuba – effectively had concussion symptoms, without ever being hit on the head.

But Bartholomew doesn't buy it.

"Sonic weapons cannot cause concussions, or white-matter tract changes – it is physically impossible," he told ABC News.

"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."

For what it's worth, the study wasn't actually conclusive on how the trauma resulted, suggesting the "clinical manifestations may represent a novel clinical entity", but explaining it remained "currently unclear if or how the noise is related to the reported symptoms".

Nonetheless, Bartholomew is dismissive of the findings, as he is of misplaced efforts by the US government to warn Americans to avoid strange noises out of "an abundance of caution".

"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'," Bartholomew said, referring to people's stereotypical tendency to arrive at unlikely medical diagnoses in the face of more plausible explanations.

"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."

Whatever the cause of the mysterious symptoms, one thing seems certain: they are not going away.

Just this week, news broke of previously unreported potential 'sonic attack' incidents in Shanghai and Beijing, adding to a list that also includes a suspected episode in Uzbekistan and what looks to have been a false alarm recently in Singapore.

On top of Cuba and Guangzhou, something certainly seems to be catching – we just don't know whether it's weaponised sound, or rampant delusions of the same.

Of course, others have suggested a less provocative explanation for the outbreaks: side effects caused by the operation of bugging equipment installed to eavesdrop on diplomatic facilities and equipment.

Whether that hypothesis (or mass hysteria) will ever be publicly acknowledged by the US government now remains unknown, given this disturbing mystery is no longer just a scientific puzzle, but an exercise in strategic brinkmanship – even if it's all been caused by something that only exists in people's heads.

"Once you say there's been an attack, you can't back up," general director of the Cuban Neuroscience Centre, Mitchell Joseph Valdés-Sosa, told The Verge earlier in the month.

"Would they be willing to recognise that they've made a mistake? They've got themselves into a corner, and they don't know how to pull out."