In July 1518, a terrifying and mysterious plague struck the medieval city of Strasbourg. Hundreds of men and women danced wildly, day after day, in the punishing summer heat. Some of them even died. In his book A Time to Dance A Time To Die, just published, a British historian of medicine based at Michigan State University has uncovered fresh evidence into why this so called dancing plague took place. The author John Waller explains what exactly the dancing plague was.