In the summer of 1518, people in Strasbourg couldn’t seem to stop dancing—some even danced themselves to death.

It all started with one woman, who stepped into the street and began moving uncontrollably. Soon, dozens of other residents followed suit, overcome with the inexplicable urge to hop, jerk, and sway. Within weeks, the outbreak had spread throughout the city, leaving officials, physicians, and religious leaders scrambling to figure out what was happening—and, more importantly, how to make it stop.

Five centuries later, the dancing plague of 1518 remains one of history ’s strangest medical mysteries. Historical records confirm that a bizarre episode turned a medieval city into a very unwilling dance floor that summer, but exactly why it happened remains a matter of debate.

A bizarre outbreak takes hold

The dancing plague began on July 14, 1518, when a woman named Frau Troffea started dancing in Strasbourg, then part of the Holy Roman Empire and now a city in northeastern France.…