In the summer of 1518, the city of Strasbourg, then part of the Holy Roman Empire, was struck by one of the strangest events in history: the Dancing Plague. It began when a woman, Frau Troffea, stepped into the street and started to dance without rest. Within days, dozens of others joined her, moving uncontrollably to music that wasn’t there. Eyewitnesses described men and women dancing until they collapsed from exhaustion. The authorities, believing the dancers were suffering from an illness...