In July 1518, a strange dancing epidemic attacked the town of Strasbourg, now a part of France. It began with a woman named Frau Troffea, who began dancing in the streets for no reason (and with no music).
Within a week, 34 others had joined in and by August there were 400. At first, musicians were brought in to facilitate the dancing, but when people’s feet were bloodied and they began to die of heart attacks, they were moved to a mountaintop where they prayed for help. Eventually, most recovered.
At the time, the dancing was attributed to a curse placed by St. Vitus, but modern historians attribute it to mass hysteria/psychosis due to stress. (Source 1 | Source 2)