In 1518 a large group of people suddenly started dancing and couldn't stop until they dropped or died

During the Dancing plague of 1518, between 50 to 400 people in Strasbourg, Alsace (modern-day France) manically danced from July to September until they apparently either had to be hospitalized or dropped dead.

From Wikipedia:

Some sources claim that, for a period, the plague killed around fifteen people per day, but the sources of the city of Strasbourg at the time of the events did not mention the number of deaths, or even if there were fatalities. — Read the rest