As the Black Death rapidly demolished vast populations of humanity, Europeans desperately sought help from the two institutions that traditionally provided them with hope and healing—the medical profession and the Catholic Church. Despite heroic attempts by many individual physicians and priests, neither institution had any success. Doctors were unable to heal victims or prevent others from catching the Black Death. Baffled by the onslaught of the Great Mortality, church officials offered little but words of comfort to those who perished or watched their loved ones die.

Human ignorance was a major cause of these failings. Medieval medicine was steeped in tradition, superstition, and false information when the Black Death struck. The Catholic Church was partly to blame, too; its restrictions on scientific research and the expression of new ideas stifled progress in efforts to understand human health. In 1300, for instance...