In July of 1518, Frau Troffea of Strasbourg, France (then part of the Holy Roman Empire), began to dance frantically in the streets. Within a month, 400 people began to do the same, eventually collapsing and dying of heart attack, exhaustion, and stroke.
Doctors at the time were at a loss. Notes from the city council revealed that the cause of the dancing was unknown, only that the victims were not dancing willingly.
Then, as suddenly as it began, in August, the Dancing Plague of 1518 was over, leaving almost 400 dead, a population baffled, and a mystery that has lasted half of a millennium.
Some have blamed the dancing plague on mass hysteria, the result of eating contaminated bread, or even religious ecstasy.
Although the plague never reappeared in France, a similar case of the frantic dancing cropped up in Madagascar in the 1840s. In both cases, the cause was never found.