Dancing Plague of 1518
Despite the July heat in 1518, Frau Troffea began silently dancing in the streets of Strasbourg, France. She'd kept up her bizarre dance marathon for nearly a week when suddenly other citizens began joining her. Soon the streets were filled with three dozen dancers. By August, an astounding 400 were shimmying and shaking nonstop all over the city.
Stunned and puzzled, doctors nevertheless proclaimed this affliction was caused by fever and recommended the dancers keep at it until their fevers were gone. The city tried to help, constructing a dance stage and bringing in a band and professional dancers, presumably to entertain the victims during their uncontrollable gyrations.
But people began collapsing from exhaustion and the heat. Some even died. The hysteria, which is well-documented in historical records, didn't end until the dancers were finally removed from the streets (they were taken to a shrine to pray for absolution).
What brought on the dancing plague? Possibly stress. Disease and famine were sweeping through Strasbourg at the time. Many people were reduced to begging. It was also a superstitious period; one common belief among Catholics was that St. Vitus could curse you with a dancing plague [sources: Andrews, Wallis].