In October 1347, when a Genoese trading ship fresh from the Crimea docked at a harbor in Sicily, dead and dying men lay at the oars. The sailors had black swellings the size of eggs in their armpits and groins, swellings that oozed blood and pus, and spreading boils and black blotches on the skin. The sick endured severe pain and died within five days of the first symptoms.

Other symptoms appeared in some of the next victims: continuous fever and spitting of blood. These victims coughed, sweated heavily, and died within three days or less—sometimes in 24 hours. No matter the symptoms, everything about the victims smelled foul, and depression and despair fell over them when they contracted the disease.

The disease, bubonic plague, was so lethal some went to bed well and died before morning; some doctors caught the illness at the patient’s bedside and died before the patient.

Borne by ships traveling the coasts and rivers, by early 1348, the plague had penetrated Italy, North Africa, France, and crossed the English Channel. At the same time, it moved across the Alps into Switzerland and reached eastward to Hungary.

In a given area, the plague wreaked its havoc within four to six months and then faded, except in larger cities. There it slowed in winter only to reappear in spring to rage for another six months. In 1349, it hit Paris again and began spreading through England, Scotland, and Ireland as well as Norway, Sweden, Denmark, Prussia, and Iceland, sometimes in chilling fashion. Off the coast of Norway, a ship drifted aimlessly offshore, finally grounding itself in Bergen. On boarding the ship, people discovered a load of wool and a dead crew.

By mid-1350, the plague had passed through most of Europe. The mortality rate ranged from 20 percent in some places to 90 percent in others. In many rural villages, the last survivors moved away, and the village sank back into the wilderness, leaving only grass-covered mounds. Overall the estimate of one medieval observer ...