Around 400 people in Strasbourg danced uncontrollably for weeks; dozens died of exhaustion. Mass hysteria, ergot poisoning, religious cult — historians still debate.
From Wikipedia
The dancing plague of 1518, or dance epidemic of 1518, was a case of dancing mania that occurred in Strasbourg, Alsace, in the Holy Roman Empire from July 1518 to September 1518. Somewhere between 50 and 400 people took to dancing for weeks. There are many theories behind the phenomenon, the most popular being stress-induced mass hysteria, suggested by John Waller. Other theories include ergot poisoning. There is controversy concerning the number of deaths.