On a mundane Tuesday in July 1518, a woman named Frau Troffea stepped into a Strasbourg street and began to dance. She didn't stop for a week. Within days the street was choked with dozens of people caught in a terrifying, involuntary compulsion, dancing until blood pooled in their shoes and their faces went blank.

This episode pulls apart the primary sources to trace the timeline of this bizarre anomaly, examine one of history's worst public health backfires, and explore the modern scientific consensus on what was happening inside these people's bodies and minds. It is a story about the physical weight of human trauma and the thin line between a rational society and a collective trance.

• How the "hot blood" diagnosis led the city council to build stages and hire musicians, pouring fuel on the fire
• The pivot to a religious ...