In 1518 in Strasbourg, a woman stepped into the street and began to dance, and within a month roughly 400 people were dancing alongside her — some of them, according to later accounts, until they collapsed and died — and no one has ever fully explained why

On a July day in 1518, in the city of Strasbourg, a woman known to history as Frau Troffea walked out of her home and into the narrow cobbled street, and began to dance.

There was no music. There was no festival, no wedding, no occasion of any kind. She simply began to move — twisting, turning, stepping in a rhythm only she could hear. Her husband, by the accounts that survive, begged her to stop. She didn’t. She danced through the afternoon. She danced as the sun went down. She danced, by some accounts, for the better part of a week, pausing only to collapse into brief, exhausted sleep before rising to dance again on swollen and bleeding feet.

And then something genuinely strange happened. Other people started dancing too.

A city that could not stop dancing

Within a week, more than thirty of Frau Troffea’s neighbours had joined her. Within a month, the number had swelled to roughly 400 people, all gripped by the same compulsion, all dancing through the streets and squares of Strasbourg.

This is not a legend. It is one of the best-documented strange events of the entire medieval and Renaissance period. The Strasbourg city council’s own minutes from that summer still survive — dry administrative notes, never meant to be dramatic, recording the spread of the dancing with bureaucratic flatness. Physicians’ notes survive. Chronicle accounts survive. Six separate contemporary sources describe the event.

What they describe is not joyful. Witnesses wrote of dancers with vacant, glassy eyes. Of arms thrashing. Of people who appeared to be in genuine distress, crying out, begging for help, and yet unable to stop moving. This was not a party. It looked, to the people watching it, like an affliction. Like something had taken hold of the body and would not let go.

According to later accounts, the toll was severe. Some chroniclers wrote that dancers collapsed and died — of strokes, of heart attacks, of sheer exhaustion. How many actually died is genuinely uncertain; the contemporary council records don’t give a number, and historians still debate it. But the image that has come down through history is stark: a city where people danced until their bodies gave out.

The cure that made it worse

The response of Strasbourg’s authorities is, in hindsight, almost unbelievable.

The city council consulted local physicians. The physicians concluded — using the medical theory of the day, which explained illness through the balance of bodily “humors” — that the dancing was caused by “hot blood,” a kind of fever. And the logical treatment for a fever, they reasoned, was to sweat it out.

So the city council decided the dancers needed to dance more.

They cleared space. They opened guildhalls. They built a wooden stage. And — this is the part that strains belief — they hired professional musicians, pipers and drummers, and paid them to play music to keep the afflicted dancing. The expense records survive. The city of Strasbourg paid, out of public funds, for the musical accompaniment to one of the most disturbing public health crises in its history.

It did not work. It made things worse. The music acted as a beacon. People who might otherwise have stayed home heard the drums, saw the dancing, and felt the pull. The crisis spread.

So what actually caused it?

This is the question that has kept the Dancing Plague of 1518 alive in the public imagination for five centuries. And the honest answer is that nobody knows for certain.

There are two leading theories.

The first is ergot poisoning. Ergot is a fungus that grows on damp rye, and it produces compounds chemically related to LSD. Eat bread made from contaminated grain and you can suffer hallucinations, convulsions, and a burning sensation in the limbs. Strasbourg had recently endured famine, which would have pushed desperate people toward spoiled food. It’s a tidy theory — but it has a serious problem. Ergot poisoning typically constricts blood flow to the extremities so severely that sustained, coordinated movement becomes almost impossible. It’s hard to dance for days on feet that ergot is actively killing.

The second theory, favoured by the historian John Waller, who wrote the definitive modern book on the event, is mass psychogenic illness — what used to be called mass hysteria. The argument runs like this. Strasbourg in 1518 was a city under extraordinary stress. Famine, disease, and crushing poverty had ground the population down. And crucially, the region had a specific local belief: that a saint, St. Vitus, could curse people with a compulsion to dance. In a population that was starving, terrified, and primed by that exact superstition, a psychological trigger in one person could — through the deep human susceptibility to suggestion — cascade through a crowd.

Neither theory fully closes the case. The ergot theory struggles with the physical reality of the dancing. The mass-hysteria theory explains the spread, but can’t be definitively proven across five hundred years.

Why the mystery endures

The Dancing Plague is not the only recorded outbreak of “dancing mania” in European history — there were others, across several centuries — but 1518 is the most thoroughly documented, and the strangest.

What makes it linger isn’t just the spectacle. It’s what it suggests about us. If the mass-hysteria explanation is even partly right, then the Dancing Plague is a demonstration of something genuinely unsettling: that under enough stress, primed by the right belief, the human mind can produce physical symptoms that spread from person to person through a crowd, with no virus and no poison involved. Just suggestion. Just fear, moving through bodies.

The dancers of Strasbourg eventually stopped. By September, the city changed tactics — the dancers were taken to a shrine of St. Vitus, given red shoes, and led through a religious rite of absolution. Whether that worked, or whether the episode simply burned itself out, is one more thing nobody can say for certain.

Five hundred years later, the records still sit in the Strasbourg archives. The street where Frau Troffea first began to dance is still there. And the central question — why — has never been fully answered.

Some mysteries don’t get solved. They just get old.