In the summer of 1518, a woman named Frau Troffea walked out of her house on a narrow street in Strasbourg and began to dance. No music played. No one danced with her. She simply started in the middle of the day, in the middle of the street, and she did not stop.

Her husband pleaded with her. She took no notice. She went on for hours into the evening until she collapsed on the cobblestones, and the next morning she was up again on swollen feet, dancing in the same place. Within days, dozens of others had joined her.

By the end of a week, the accounts say, more than thirty men and women were dancing in the streets. Within a month, some chronicles put the number at four hundred. They danced in rings, hand in hand, their eyes open but seeing nothing, for hours at a time. When they fell, their bellies swelled hard and they groaned as if dying.

Bystanders wound cloths tight around their waists for relief, and where cloth ran out, they thumped and trampled on the parts affected. People stood on them, and the dancers got up afterward and went back into the ring. The city reacted as best it could. The physicians were called in, and they gave a diagnosis that was unusually medical for the time: the dancers suffered from overheated blood.

Reasoning that the dance itself was the body working the heat off, the council ordered the dancers to dance more. Two wooden platforms were built in the markets. Guild halls were turned into dance floors. The council hired musicians—drummers, pipers, fiddlers, men with horns—to play for them, on the theory that music would help the fit run its course faster.

Healthy dancers were brought in to hold the afflicted up and keep them moving. It made things worse. The stages advertised the dance to the entire city. Crowds gathered to watch, and many of them went home and started dancing themselves.

The council reversed itself. The stages came down, the musicians were sent home, and a ban went out on public music and dancing until the end of September. The exception was written by the city’s chancellor, Sebastian Brandt, the author of The Ship of Fools, who had watched his own city fall into a frenzy. If honorable persons wished to dance at weddings, he wrote, they might do so using stringed instruments, but they were on their conscience not to use tambourines and drums.

No drums. A whole city told that the beat itself was the danger. The dancers who could no longer stand were loaded into wagons and driven three days west to a shrine of St. Vitus near Saverne.

There, the priests had a routine. Each dancer was laid beneath a wooden image of the saint and given a small cross to hold. Red shoes were put on their feet and sprinkled with holy water. Crosses of consecrated oil were painted onto the soles and uppers.

Incense was burned, Latin was recited, and then they were left. Many of them came out of the trance there. Within about a week of the wagons starting, the stream of new dancers slowed. By late August it was a trickle.

By early September it was over. Nobody in Strasbourg that summer had a word for what they had watched. Some called it a plague. Some called it the devil.

What they had seen was not new, though for them it was unprecedented. Their grandparents had known the story of Kölbigk, a village in Saxony where, on Christmas Eve of the year 1021, eighteen people had begun a ring dance in the churchyard while the priest tried to say mass inside. He had cursed them and asked God to let them dance for a whole year without ceasing. According to the story, God did.

They danced through the winter and the spring and the summer and back into winter again, neither eating nor sleeping, until the ground wore away beneath them and they danced knee-deep in a pit of their own making. When the priest’s son tried to pull his sister from the ring, her arm came away in his hand, and she kept dancing, and the stump did not bleed. A monk recorded that a man who had been there arrived at his abbey twenty-three years later, trembling. That story sat in the ground for three centuries, kept alive by preachers as the standing example of what happened to people who danced on a holy day.

The circumstances in 1518 had prepared the ground. The Black Death had come through the Rhineland twenty-five years earlier and taken a third of the population, so that people in their thirties had watched it as children. The Rhine had flooded in the winter of 1373 into 1374, rising thirty-four feet in some accounts, pouring over the town walls, leaving carcasses of horses floating through the streets. The crops on the flood plain were gone.

Bread was short. Then, around the feast of St. John the Baptist at the end of June, the dancing started. St.

John’s Eve was already a night for bonfires, and there was a belief in the region that St. John could send a dancing sickness on people who displeased him, just as St. Vitus could further south. Strasbourg sat on the Rhine.

The outbreaks of the 1370s had moved along that river and the Moselle—Aachen, Cologne, Liège, Utrecht, Tongeren, Metz, Trier, Ghent—and wherever the dancers arrived, locals joined them. At Tongeren, the dean wrote that in their songs the dancers uttered the names of devils never before heard of, and he was a man whose profession involved knowing the names of devils. The dancers took over religious houses. Processions were held on their account.

The clergy came to believe they were a sect and hurried their exorcisms. At Metz, the streets were said to be filled with dancers; peasants left their plows, mechanics their workshops, housewives their houses. Bands of beggars learned to imitate the convulsions and went from place to place living off the spectacle until the towns drove them out. The explanations for what happened to those towns went back and forth for centuries.

Some argued for ergot, a fungus that grows on damp rye and produces alkaloids related to LSD, causing convulsions, hallucinations, and mania. But ergot cuts off the blood to the legs. People poisoned by it convulse and see things, and their feet turn black and fall off. They do not dance in coordinated rings for days on end.

Others argued for a real trance, driven by stress and fear of damnation, dancing in the shape that belief had cut for them. Both could be held at once: a group of frightened people who believed a curse was on them, walking from shrine to shrine, looking for the saint who could lift it, picking up more frightened people as they went until the road itself was full of them. The outcome at Strasbourg left a thin record. The council minutes do not mention any deaths.

Later writers claimed the dead came to fifteen a day at the peak, but those accounts were written long afterward. What is certain is that people danced for days on broken feet in the height of summer without eating. The urgencies of the moment passed. A ban on public dancing held until the end of September, and by early September the last of the dancers were taken up to the chapel to pray.

The city went back to worrying about bread. The dancers themselves left almost nothing behind. Their names are mostly lost. Frau Troffea survives because Paracelsus, the physician, came to Strasbourg eight years afterward and wrote about her with contempt.

He thought she had started it out of spite to annoy her husband, and that the other women who followed her were doing the same to their own husbands. Whatever the truth of that, she had the last quiet word: her name is in the record, and that is most of why we have it. The dancing stopped. The wagons stopped running up the road to Saverne.

The red shoes went back into storage at the shrine. The city absorbed it as it had absorbed the flood and the plague and the failed harvests, and the chronicles noted it between the price of grain and the doings of the bishop.