This book by German author I. F. C. Hecker discusses the "Dancing Malady," or "St. John's Dance," that occurred in the Middle Ages directly preceeding the Black Death. Doctors are still mystified by the affliction, but primarily consider it a collective hysterical disorder which may have been brought on by the stresses of the time, since itℓmostly affected the laboring class during the Middle Ages.ℓ