In July 1518, a woman in Stasbourg, France began dancing fervently in the streets. Soon, somewhere between 50 and 400 joined her in dance and it's not known why. Some reports mention numerous deaths at the hands of this "plague" while others mention no deaths at all. Originally, these "dancing plagues" were considered to be caused by curses or some kind of possession. Modern theories posit food poisoning, spider bites, or stress-induced mass hysteria.
I'm gonna go with spider bite here.
