Here’s what you’ll learn when reading this story:

• Many episodes of a “dancing plague”—incidents when people dance uncontrollably past the point of injury—can be found throughout the Middle Ages, with none bigger than the Dancing Plague of 1518 that impacted 400 people in Strasbourg.
• At the time, most people believed the dancing to be a curse by saints or perhaps the symptoms of an “overheated body,” though a few expected it might be caused by stress.
• Today, leading historians and psychologists argue that these dancing episodes were likely examples of mass psychogenic illness induced by stress but further supported by medieval beliefs that dancing curses were a real malady.

In July 1518, Frau Troffea—a resident of the city of Strasbourg, then part of the Holy Roman Empire (now in France)—stepped outside her home and began dancing. She wasn’t dancing in anticipation of some local summer tradition or to the beat of some irresistible tune. No, Troffea reportedly began dancing uncontrollably and, what’s worse, completely unwillingly. Seeing as the human body can’t dance without end, she eventually collapsed from exhaustion only to recover her strength and begin dancing anew—but then things got really weird.

The event was best chronicled by the famous alchemist Paracelsus, who visited the city years after the unexplainable event and recorded what happened in his Opus Paramirum. As Troffea continued to dance for days on end, others—also uncontrollably and unwillingly—began joining her. Within four to six days, Troffea was whisked away to a nearby shrine of Saint Vitus (whom they believed had cursed her) in the hopes of curing her illness, but the strange mania’s hold was unshakeable. Within a few days of Troffea’s first manic choreography, some 30 other people had also fallen victim to this mysterious “dancing plague.”

The local privy council, concerned by the unexplainable dancing, concluded that the disease may be caused by “a natural disease, which comes from overheated blood” and decided that dancing was the best medicine. The town hastily constructed dance halls, but to no avail, as by the end of August, some 400 people in the region were dancing past the point of injury. Contemporary chroniclers cite as many as 15 people dying per day, while modern historians put the cumulative number of deaths at around 100. Finally, as the calendar turned toward September, the mania appeared to abate and life returned to normal.

As strange as this dancing display sounds, incidents of uncontrolled dancing can be found throughout the Middle Ages and even during parts of the Dark Ages. In 1020, for example, some 18 peasants inexplicably danced outside of a church in Bernburg, Germany, and in a scene straight from a horror film, children danced uncontrollably from Erfurt to Arnstadt in 1237, an event that possibly inspired the legend of the Pied Piper of Hamelin. This phenomenon, which became known as St. Vitus’ dance (believed to be a curse placed on the saint), all but disappeared by the 17th century. It was actually Paracelsus who originally posited the idea that “the saints have nothing to do with this disease” and instead suggested that the uncontrolled choreography likely arose from anxiety.

Some 500 years after the event, experts aren’t completely sure what exactly caused these events. While some suggest that psychoactive ingredients of mushrooms might’ve induced a mass psychosis, others suggest that drugs alone couldn’t explain the extended duration of the dancing. John Waller, an associate professor of history at Michigan State University and author of the book The Dancing Plague: The Strange, True Story of an Extraordinary Illness, believes that these dancing fits might have been a mass psychogenic illness caused by stress that was further supported by prevalent superstitious beliefs that curses, such as St. Vitus’ dance, actually existed. These factors could also explain why most historical evidence of mass dancing episodes is limited to the period between the 13th and 17th centuries.

“Modern anthropology and psychology also reveal how beliefs and expectations can shape the individual's experience of dissociation,” Waller wrote in a 2009 article for The British Psychological Society. “That the dancing plagues were reliant on cultural belief-systems is apparent from the fact that they were concentrated in just those communities where we know there to have been a pre-existing belief in the possibility of dancing curses being sent down from Heaven or Hell.”

So then maybe these eerie displays of dancing weren’t so much a plague of the body, but rather, of the mind.