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.
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. There do not appear to be any sources contemporaneous to the events that make note of any fatalities.
It all started when a woman started dancing wildly in the middle of the street. More and more people joined, mostly young women. Historians agree that the dancing plague took place, but the cause is still unknown.
Possible explanations include demonic possession, overheated blood, spider bites, food poisoning caused by the toxic and psychoactive chemical products of ergot fungi, or a mass psychogenic illness.
I haven't found any evidence that music was playing during the dancing plague, so I assume that the afflicted people were dancing like victims of demonic possession in silence. This mental image makes the whole thing a million times creepier.