Today, scientists understand that the Black Death, now known as the plague, is spread by a bacillus called Yersina pestis. (The French biologist Alexandre Yersin discovered this germ at the end of the 19th century.) They know that the bacillus travels from person to person pneumonically, or through the air, as well as through the bite of infected fleas and rats. Both of these pests could be found almost everywhere in medieval Europe, but they were particularly at home aboard ships of all kinds–which is how the deadly plague made its way through one European port city after another. Not long after it struck Messina, the Black Death spread to the port of Marseilles in France and the port of Tunis in North Africa. Then it reached Rome and Florence, two cities at the center of an elaborate web of trade routes. By the middle of 1348, the Black Death had struck Paris, Bordeaux, Lyon and London.
Today, this grim sequence of events is terrifying but comprehensible. In the middle of the 14th century, however, there seemed to be no rational explanation for it. No one knew exactly how the Black Death was transmitted from one patient to another–according to one doctor, for example, “instantaneous death occurs when the aerial spirit escaping from the eyes of the sick man strikes the healthy person standing near and looking at the sick”–and no one knew how to prevent or treat it. Physicians relied on crude and unsophisticated techniques such as bloodletting and boil-lancing (practices that were dangerous as well as unsanitary) and superstitious practices such as burning aromatic herbs and bathing in rosewater or vinegar.
Meanwhile, in a panic, healthy people did all they could to avoid the sick. Doctors refused to see patients; priests refused to administer last rites. Shopkeepers closed stores. Many people fled the cities for the countryside, but even there they could not escape the disease: It affected cows, sheep, goats, pigs and chickens as well as people. In fact, so many sheep died that one of the consequences of the Black Death was a European wool shortage. And many people, desperate to save themselves, even abandoned their sick and dying loved ones. “Thus doing,” Boccaccio wrote, “each thought to secure immunity for himself.”