The Middle Ages was no time to start a rave—but that didn’t stop Frau Troffea. On July 14, 1518, she stepped onto the streets of Strasbourg, France, and, although there was no music playing, began to boogie uncontrollably. Troffea danced for three straight days, and by the time she was tied up and hauled away, more than 30 other people had joined in. Within a month, 100 people were frantically jigging—and none of them could stop.

This was no ordinary dance party. Hyperventilating and hallucinating, most of the dancers seemed to be totally unconscious. Seldom able to stop for food or rest, some literally danced until they dropped dead of heart attack, stroke, or exhaustion. Local physicians ruled out supernatural causes, blaming the “dancing plague” on “hot blood.” They also decided that the best course of action was to encourage the delirium to be danced out. Authorities erected a stage and hired musicians, but the plan backfired: It just encouraged more people to dance.

This wasn’t the first time a European village had been plagued by “dancing mania.” The first outbreak had occurred in the seventh century, and cases sporadically struck every few decades. No country was immune: Italy, France, Holland, and Germany all suffered. The Strasbourg plague, however, was the worst. It struck 400 people and lasted until September, when it suddenly stopped as mysteriously as it began.

The cause of these dancing plagues? Some blame ergot, a poisonous mold found on damp rye thats related to LSD. Others accuse Sydenham’s chorea, a disorder linked to strep throat and rheumatic fever that causes fluid, dance-like twitches. Medical historian John Waller makes a more psychological diagnosis: stress-induced mass psychosis. Sixteenth-century Europe was not a great time to be alive: The Black Death loomed large, smallpox and syphilis were raging, and famine was everywhere. All the ingredients for mass hysteria were there, just waiting for someone to snap.