Why does mass hysteria spread?

Here is a scene that has played out, in one form or another, hundreds of times across the last century. A student in a packed assembly faints. Then another. Within the hour, a dozen more are dizzy, breathless, their hearts pounding, some collapsing, some vomiting. The school is evacuated. Hazmat teams sweep the building; the water, the air and the cafeteria food are all tested. And every single test comes back clean. Nothing was in the room that could have made anyone ill. And yet the illness was completely real. So what, exactly, moved from one person to the next?

01 · The patternReal symptoms, no physical cause

The phenomenon has an unfortunate old name, mass hysteria, and a growing pile of better ones. Strip away the drama and the pattern is remarkably consistent. A group of people, usually under real stress and often crammed together (a school, a factory floor, a barracks), starts to fall ill with genuine symptoms: fainting, nausea, dizziness, headaches, twitching, trouble breathing or walking. The symptoms are real. The doctors’ inability to find a germ, a gas or a poison is also real. Both things are true at once, and holding them together is the whole key to understanding it.

Crucially, this is not rare, and it is not a relic. Documented outbreaks stretch from possessed medieval convents to modern high schools, and they keep happening. What links them is not a place or an era. It’s a set of conditions.

02 · The laughterTanganyika, 1962

The strangest entry in the file began at a girls’ boarding school in the village of Kashasha, in what was then Tanganyika, in January 1962. Three girls started laughing and could not stop. It spread. Within weeks, most of the school was caught up in fits of uncontrollable laughing and crying, along with restlessness, pain and fainting. The school had to close, and as the pupils went home, it spread to their villages, then to other schools.

By the time it faded, the outbreak had touched around a thousand people and shut 14 schools. The details are worth getting right, because the story is usually inflated: nobody laughed non-stop for months. Individual attacks lasted anywhere from a few hours to about a fortnight, and the epidemic as a whole waxed and waned over roughly eighteen months. Doctors examined the afflicted and found nothing physically wrong. What they were watching was a stressed population, in a newly independent country, passing an involuntary reaction from one person to the next.

03 · The June bugHow suggestion does the work

That same year, on the other side of the world, gave us the textbook case. In June 1962, workers at a textile mill in Spartanburg, South Carolina, began falling ill with nausea, dizziness and numbness. The story going round the floor was that a strange insect, a “June bug”, had arrived in a shipment of English cloth and was biting people. Entomologists were called in. They combed the mill and found no bug capable of doing any such thing.

Sociologists Alan Kerckhoff and Kurt Back studied the outbreak in detail, and their findings are the clue to the whole phenomenon. Of the roughly sixty affected workers, the overwhelming majority were women, most worked the same overloaded shift, and a striking number only reported symptoms after seeing others fall ill or hearing about it. The illness spread along lines of friendship and sight. The bug was a story the anxiety told itself. The stress, the exhaustion and the suggestion were the real bite.

04 · The triggerA smell, a rumour, and the nocebo effect

Often there is a plausible-but-wrong explanation that lights the fuse, and it does not need to be true to work. It only needs to be believed. In the spring of 1983, across the West Bank, roughly 943 people, about seven in ten of them schoolgirls aged 12 to 17, reported fainting and dizziness in wave after wave. In the tense atmosphere, many were convinced they had been poisoned by gas. Investigators, including the American CDC, concluded the outbreak was largely psychogenic, perhaps set off in one school by the faint smell of low levels of hydrogen sulfide, a real but harmless whiff that a frightened population read as an attack.

This is where mass psychogenic illness shakes hands with the nocebo effect, the placebo effect’s dark twin. If expecting a sugar pill to help can genuinely ease your symptoms, expecting a smell to poison you can genuinely make you ill. A rumour of gas, an odd odour, a whispered word of contamination: the belief that you have been exposed is enough for the body to produce the sickness to match. Multiply that across a scared crowd, and you have an epidemic with nothing in the air.

05 · The nameWhy "hysteria" is a bad word for it

The word “hysteria” comes loaded, and every ounce of the load is wrong. It descends from the Greek for womb, and it drags along an old, ugly assumption: that the (usually female) sufferers are irrational, attention-seeking, or simply making it up. But the evidence says the opposite. The symptoms are involuntary, the people are ordinarily healthy, and the process is a normal group response to stress, not a defect in anyone’s character.

So researchers Robert Bartholomew and Simon Wessely have argued for a cleaner term: mass sociogenic illness. “Socio” because the driver is social, and “genic” because it genuinely generates real symptoms. Bartholomew has pointed out that the phenomenon has been relabelled dozens of times over the years, a sign of how uneasily the field has sat with its own vocabulary. The relabelling matters. If you call it hysteria, you blame the victims. If you call it sociogenic illness, you can actually see what is happening: real bodies reacting to a shared, and mistaken, sense of threat.

06 · The new vectorWhen the sufferer is on a screen

For centuries, the illness needed proximity. You had to see or hear an affected person for the anxiety to jump to you. That constraint is now dissolving. Between 2011 and 2012, in Le Roy, New York, around nineteen students (almost all teenage girls) and one older woman developed tics, twitches and verbal outbursts. As with every case before it, the environmental and infectious testing came back empty, and doctors identified a functional condition, conversion disorder. But something was different this time: the students posted videos of their symptoms, and the reports spread through Facebook and YouTube. Researchers flagged it as possibly the first such outbreak partly propagated by social media, where you no longer had to be in the room to catch the fear.

07 · The TikTok chapterAn illness that spreads through a phone

Then came the sharpest test of that idea. From around 2019, and accelerating through the pandemic lockdowns, clinics around the world reported a sudden surge of young people, mostly teenagers, arriving with abrupt, severe, functional tic-like movements and outbursts. Many had been watching popular tic-related videos beforehand. In 2022, a German team led by Kirsten Müller-Vahl described this as a genuinely new thing: a mass sociogenic illness spread entirely through social media, with a hugely popular YouTube creator serving as a kind of “virtual” index case whose specific gestures and phrases turned up, uncannily, in patients who had never met.

It is a compelling account, and it fits the century-old template with one dramatic update: the vector is now a screen, not a shared room. But it must be hedged. The framing is genuinely contested, other researchers caution against over-diagnosing every young person’s tics as socially induced, and the picture is still being worked out. What is not in doubt is that the patients’ symptoms are real and involuntary. Whether “social media illness” is the right label is the part still being argued.

08 · The payoffSo why does it spread?

Pull it all together and the answer is almost disappointingly human. Mass psychogenic illness spreads because we are social animals wired to take our sense of danger from the people around us. Put a group under real strain, give them a plausible reason to feel threatened, a bite, a smell, a rumour, a viral video, and let them watch one of their own genuinely fall ill, and the anxiety does the rest. Their bodies are not lying. They are doing exactly what fear tells bodies to do.

That’s the thread running from a Strasbourg street in 1518, where hundreds danced until they dropped, straight through to a teenager’s bedroom in 2021. Not a poison, not a performance, and certainly not weakness. Just a genuine illness whose true cause is an idea, moving through a frightened group faster than any germ could manage. The symptoms are real every time. It’s the danger that isn’t there.