I just can't stop choreomaniaing! St Vitus' Dance plagues my brother and I thanks to my paternal side. I was just reading about genetic theories and came across some of the best Wiki paragraphs ever:

Dancing mania (or choreomania) was a social phenomenon that occurred primarily in mainland Europe between the 14th and 18th centuries; it involved groups of people, sometimes thousands at a time, who danced uncontrollably and bizarrely, seemingly possessed by the devil. Men, women, and children would dance through the streets of towns or cities, sometimes foaming at the mouth until they collapsed from fatigue.

One of the first major outbreaks was in Aachen, Germany, on June 24, 1374; the populace danced wildly through the streets, screaming of visions and hallucinations, and even continued to writhe and twist after they were too exhausted to stand.

Having occurred to thousands of people across several centuries, dancing mania was not a local event, and was, therefore, well-documented in contemporary writings. More outbreaks were reported in the Netherlands, Cologne, Metz, and later Strasbourg (Dancing Plague of 1518), apparently following pilgrimage routes.

And the Dancing Plague of 1518 is my brother's favourite bizarre historical event. Let the dancing commence.