Ah, here's one of those weird historical nuggets that you expect from a blog entitled "Things I've Learned from Wikipedia."
Submitted for your approval: the year is 1518. The place: Strasborg, France. A woman begins dancing uncontrollably in the street, continuing for days without rest. Within a month, she has been joined by 400 of her closest friends and family. Local officials attempt to cure the epidemic by opening guildhalls and hiring musicians, in hopes that that encouraging the dancers will allow them to stop. But unbeknownst to these medieval townsfolk, they are just about to foxtrot themselves into... the Twilight Zone.
Seriously though, a lot of people died.
The article includes several theories as to what caused the Dancing Plague. Mass psychogenic illness seems to be the most probable explanation, but there is another theory:
"Historian Nikhil Murthy of Emory University believes that the epidemic can largely be attributed to the accidental synthesis of MDMA. Although MDMA was not formally synthesized in a laboratory until 1912 by Anton Köllisch, an accidental mixture could easily have been concocted by subjecting Safrole, a commonly available plant in the area, with a basic substance."
DARK AGES RAVE! WHOOOOOOOOOOO! The next time I go to a Renaissance Faire, I'm bringing glowsticks and candy necklaces!