A few weeks after the 2016 presidential election of Donald Trump, several people working for the U.S. Embassy in Cuba fell mysteriously ill. Some lost their hearing. Some had headaches and a pain in one ear. Others reported feeling dizzy or nauseous, having trouble focusing, or feeling fatigued. Later, some would have a hard time concentrating, remembering things, sleeping, and even walking.

These symptoms were “medically confirmed,” as the State Department’s medical director Charles Rosenfarb put it, and brain scans were said to show abnormalities in the victims’ white matter, which transfers information between brain regions. The illnesses were believed by the government to be “health attacks,” carried out by a foreign power, though as Todd Brown, assistant director at the Bureau of Diplomatic Security, told the Senate Foreign Relations Committee, “investigative attempts and expert analysis failed to identify the cause or perpetrator.”

Nonetheless, investigators concluded the illnesses, which ultimately affected 24 people, were likely the result of a “sonic device.” This conclusion seems to be primarily due to the fact that some diplomats reported hearing a high-pitched noise in their homes and hotel rooms.

Despite a lack evidence for such a weapon, or any known way it could affect white matter, the sonic weapon theory proved irresistible for both media outlets and for Cuba hawks like Sens. Marco Rubio and Bob Menendez, both of whom immediately transformed the sonic weapon into a handy political weapon.

In the months following the “attacks,” new diplomats arriving in the country were warned of this sonic danger. Embassy employees were played a recording of what was thought to be the sound so they knew what to listen for. Soon, people at the Canadian Embassy in Cuba began reporting symptoms similar to what the Americans had experienced, as did a few tourists there. A husband and wife at the U.S. Embassy in Uzbekistan became ill as well. Whatever it was, it seemed to be spreading.

There is increasing recognition that these epidemics of hysteria, which usually mirror prominent social concerns, present real individual and public health problems.

With no details, no motive, and no plausible explanation for what kind of weapon this might be, doubts began to surface. The FBI investigated and reportedly found there had been no such attack (though it suggested maybe it was a “viral” weapon). Sen. Jeff Flake also cast doubts on the sonic version of events. A handful of skeptical stories began to appear amid the more alarming ones, suggesting this might be what in the past was known as, “mass hysteria,” but which is now referred to as “mass psychogenic illness,” or a “collective stress response.” (These include things like the twitching girls in Le Roy, New York, in 2011; the 600 paralyzed girls in Mexico in 2007; and the Belgian Coca-Cola scare of 1999, which affected 100 students with more than 900 others reporting a related symptom, and costing the company somewhere between $103 million and $250 million.)

Epidemics of this sort are well-known in the scientific literature. Robert Bartholomew, a New Zealand–based medical sociologist and the co-author of Outbreak! The Encyclopedia of Extraordinary Social Behavior, Mass Hysteria in Schools: A Worldwide History Since 1566, and other books on the subject, has collected a database of some 3,500 cases. While the precise mechanisms are difficult to pinpoint, and the diagnosis is always controversial, there is increasing recognition that these epidemics of hysteria, which usually mirror prominent social concerns, present real individual and public health problems.

Yet many people still assume victims of such phenomena are simply faking or imagining their symptoms. In the Senate hearings on the attacks, Sen. Rubio asked Rosenfarb whether he thought this was, “a case of mass hysteria, that a bunch of people are just being hypochondriacs and making it up?”

This was a loaded question, with Rubio deploying the term mass hysteria as a means of dismissing this possibility altogether. But Rubio’s assumption—that a mass psychogenic illness is the same as faking or hypochondria—is wrong, as was his dismissal of the idea that this might explain the illnesses in Cuba. Indeed, mass psychogenic illness is likely the best explanation for these illnesses. According to Bartholomew, if you removed the word concussion from discussion of what happened there, you’d have a “textbook case” of mass psychogenic illness, in everything from its symptoms to its spread.*

“There’s no evidence whatsoever that this was caused by a sonic device,” Bartholomew says. “It is physically impossible to have brain damage caused by an acoustical device. And most of those symptoms are not symptoms of sonic weaponry.” Anxiety and nausea, he notes, can be caused by both mass psychogenic illness and acoustic weapons, but the noise would have to be incapacitating and high volume. None of the other symptoms reported in Cuba are associated with an acoustic assault.

And what’s more: “This is a small, close-knit community in a foreign country that has a history of being hostile to the United States,” he says. “That is a classic setup for an outbreak of mass psychogenic illness.”

History is filled with cases of “sounds” making people ill. In Kokomo, Indiana, locals have been plagued since 1999 by a low frequency hum, which one resident said caused, “short-term memory loss, nausea, and hand tremors.” In Taos, New Mexico, a similar sound causes resident “sleep problems, earaches, irritability, and general discomfort,” by one account. Similar hums are reported in Bristol, England, and Windsor, Ontario. In 1989, a “Low Frequency Noise Sufferers Association” was formed in London. The people reporting illness from the noise produced by wind turbines have given the phenomenon its own name: wind turbine syndrome.

Unfortunately, it is also possible to lose your hearing without being attacked by a secret weapon. The Handbook of Clinical Neurology volume on Functional Neurological Disorders lists “nonorganic hearing loss” in its chapter on “Functional Auditory Disorders,” alongside conditions like musical hallucinations, misophonia (“hatred of sound”), “acoustic shock” from a sudden noise (symptoms include “pain in or close to the ear,” tinnitus, balance problems, hypervigilance, and sleep disturbance), and others. In Germany, there is a common condition called Hörsturz, which is a sudden loss of hearing related to stress. In 1973, at a nursing school in Papua New Guinea, there was an epidemic in which students were struck deaf, among other symptoms, with no apparent external cause.

“It’s very easy to manipulate people’s physical well-being through giving them expectations about sound,” says Keith Petrie, who researched the power of the mind in relation to wind turbine syndrome. When Petrie and colleagues exposed people to both infrasound and sham infrasound (silence), they found it wasn’t the sound itself, but their expectations—or what’s known as the nocebo effect—that produced adverse physiological reactions. Witnessing another person with symptoms can create an even stronger response, as can the perceived cause.

“When we gave them a plausible, biological explanation,” says Petrie, “it increased their symptoms the next time they were exposed to sound. When we gave them a nocebo explanation—and both explanations were equally credible—their symptoms decreased.”

On the surface, studies like this make it easy to agree with the Marco Rubio line of thinking that sufferers are just faking it. But the people who were told there was a medical reason for the hearing loss are not just imagining the resulting symptoms—they are physiologically real, “medically verifiable,” and cause deep distress, even if they resolve quickly, as most do.

“People suffering from mass psychogenic illness are not hypochondriacs and they’re not all making it up,” says Bartholomew. “It is a real condition with real symptoms. It could happen to anybody.”

There is real crossover between the condition’s mental origin and physical manifestation.

Research into the nocebo effect has been hampered by the ethics of subjecting people to it, but a picture of the mechanisms is emerging. And one important factor is “abnormally focused attention,” as neurologist Jon Stone puts it.

“As human beings, we’re more prone to these phenomena than we like to think,” says Stone, co-editor of Functional Neurological Disorders. “The rate of functional symptom experiences in the general population is very high. People have these symptoms a lot and just normalize them. We’re never very far from a functional disorder.”

What were once known as conversion disorders (meaning the conversion of a mental problem into a physical one) are now referred to as functional disorders. The old terms like psychosomatic or even psychogenic imply a purely mental origin, but the current parlance reflects the more complicated picture, that there is real crossover between the condition’s mental roots and physical manifestation. A “functional disorder means something has gone wrong with the network, the connections, the pathways, as opposed to the physical structure of the brain. And when these functions go wrong, normal sensations like tiredness, dizziness, or pain can grow much worse and become persistent.

One of the findings in Cuba that reporters seized on was the assertion that victims had suffered some kind of head trauma. As Rosenfarb put it, there were “clinical findings of some combination similar to what might be seen in patients following mild traumatic brain injury or concussion.” Here, he appears to be talking about abnormalities in the patients’ white matter, but a concussion isn’t the only thing that can have that effect. White matter changes with experience and learning, and becomes more robust in response to using a given pathway repeatedly. If those pathways are related to a disorder, it may appear in “diffusion tensor imaging” scans as anomalies.

“Diffusion tensor imaging,” says Stone, “is a technique that shows abnormalities in patients not only with minor brain injuries, but also with chronic pain, anxiety, depression, you name it. This is not a mark of brain injury. It’s a mark of brain dysfunction. It’s evidence that they’re ill.”

One problem in understanding the reality of a functional disorder is that most of us, when we are ill, look for a single cause, a simple chain of events that starts with an event, or a germ, and ends with our own misery. But functional disorders don’t work in this linear fashion. They are recursive and multifactorial, a feedback loop between our expectations, emotions, and physiology. According to Mark Hallett, a senior investigator at the National Institute of Neurological Disorders and Stroke, a picture of how they work is beginning to emerge, in a sense that, “when the so-called limbic system of the brain [the part of the brain that drives instinct, mood, and emotion] is overactive, it might induce the different symptoms that arise.”

Which is to say that if we fixate on our naturally occurring experiences and feelings, they can become amplified, particularly if the limbic system is overactivated by fear and anxiety. This creates a kind of loop between mind and body that it can be difficult to get out of, and which can make these conditions difficult to treat.

“It’s very powerful,” says Petrie. And often underestimated. “From the comments I read by the physician associated with the [embassy attacks], it was interesting how he dismissed this explanation” Petrie says. “He didn’t seem to understand how easily this can happen.”

Most people don’t. That includes just about everyone involved in the Cuban attacks. Mass psychogenic illnesses are not as intuitive to grasp as cold or a flu, but they are just as serious, and should be treated as such. In Cuba, they have not been. Instead, a fixation on secret weapons has obscured a real illness with real consequences, one which can not only be “medically verified,” but which regularly afflicts people across the world, and to which anyone with a functioning brain is vulnerable.

*Correction, Feb. 8, 2018: In the original version of this story, the author stated that white matter tract changes were a sign of mass hysteria, when in fact that connection has not yet been researched.