In July 1518, a woman stepped into a narrow street in Strasbourg and began to dance. Nobody was playing music. She danced alone, in silence, for hours. Then for nearly a week. Her name was Frau Troffea, and she could not stop.

Within days, thirty-four people had joined her. Within a month, as many as four hundred residents of Strasbourg were dancing in the streets, day and night, without rest. They were not celebrating. Their faces were twisted in pain and terror. They begged for it to stop. They could not stop.

The city council convened in emergency session. They consulted physicians. They consulted priests. The physicians concluded that the affliction was caused by "hot blood" and that the cure was more dancing. The council's official response was to clear two guild halls and a grain market, build a wooden stage, and hire professional musicians and dancers to accompany the afflicted, on the theory that if they danced hard enough, the fever would break.

It did not break. It got worse. The music accelerated the frenzy. More people joined. Contemporary accounts describe dancers collapsing from exhaustion, their shoes soaked with blood from feet that had not stopped moving in days. Some reportedly suffered strokes and heart attacks, though the city records do not confirm specific deaths.

The authorities reversed course. They banned public dancing and music. They removed the afflicted to a mountaintop shrine, where they were given blessed crosses and red shoes and prayed over until the compulsion faded. By early September, the plague had burned itself out as mysteriously as it had begun.

No one has ever fully explained what happened. The region was in crisis: failed harvests, smallpox, syphilis, and a population living under what the historian John Waller described as extraordinary levels of psychological stress. Waller's leading theory is mass psychogenic illness, a phenomenon in which extreme collective anxiety manifests as involuntary physical symptoms. It was not the first time. A similar dancing mania had swept the Rhine region in 1374.

Frau Troffea was the first to dance and among the first to recover. What happened to her afterwards, no one recorded. We know her name only because the Strasbourg city council wrote it down in the minutes of an emergency meeting about a woman who would not stop dancing.

Four hundred people followed her. Nobody knows why.

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