. . There was a crowd of us, now we are almost alone.”

In the Islamic Middle East and North Africa, mortality rates also were in the one-third range. To the Muslim historian Ibn Khaldun, it seemed “as if the voice of existence in the world had called out for oblivion.” In China the presence of chronic war makes it difficult to assess plague mortalities, but between 1200 and 1393 the population of the country fell 50 percent, from about 123 million to 65 million. Today a demographic disaster on the scale of the Black Death would claim 1.9 billion lives.

The Black Death would be an extraordinary accomplishment for any wandering sickness, but it is an especially extraordinary one for a sickness not even native to humans. Plague is a disease of rodents. People are simply collateral damage, wastage in a titanic global struggle between the plague bacillus Yersinia pestis and the world’s rodent population.* Y. pestis’s natural prey are turbots, marmots, rats, squirrels, gerbils, prairie dogs, and roughly two hundred other rodent species. For the pathogen to ignite a major outbreak of human disease on the scale of the Black Death, a number of extraordinary things had to have happened. And while we will never know what all of them were, from about 1250 onward, social, economic, and perhaps ecological changes were making large parts of Eurasia an increasingly unhealthy place to live.

One new risk factor was increased mobility. Along with facilitating international trade, the Mongol unification of the steppe brought merchants, Tartar officials, and armies into proximity with some of the most virulent, and heretofore isolated, plague foci in the world. Rodents (and more to the point, their fleas) that once would have died a lonely, harmless death on a Gobi sand dune or Siberian prairie now could be transported to faraway places by caravans, marching soldiers, and riders of the Mongol express, who could travel up to a hundred miles a day on the featureless, windswept prairies of the northern steppe.

Environmental upheaval may also have played a role in the origin of the plague. Like a vain old matinee idol, Y. pestis is fond of ecological drum rolls. In the mid-sixth century, during the pestilence’s first (documented) visit to Europe, the Plague of Justinian, there were reports of blood-colored rain in Gaul, of a yellow substance “running across the ground like a shower of rain” in Wales, and of a dimming of the sun everywhere throughout Europe and the Middle East. “We marvel to see no shadow on our bodies at noon, to feel the mighty vigor of the sun’s heat wasted into feebleness,” wrote the Roman historian Flavius Cassiodorus.

Similar, if less flamboyant, accounts of environmental instability appeared in the decades prior to the Black Death. In the West as well as the East, there were reports of volcanic eruptions (Italy), earthquakes (Italy and Austria), major floods (Germany and France), a tidal wave (Cyprus), and swarms of locusts “three German miles” long (Poland). However, since the medieval world viewed natural disasters as portents and expressions of Divine Wrath, these accounts have to be read with caution. Undoubtedly, many of the calamities described by European—and Chinese—chroniclers were invented or exaggerated beyond recognition after the fact to provide the Black Death with a suitably apocalyptic overture.

That said, tree ring data indicates that the early fourteenth century was one of the most severe periods of environmental stress in the last two thousand years—perhaps due to unusual seismic activity in the world’s oceans. And modern experience shows that ecological upheaval in the form of droughts, floods, and earthquakes can play a role in igniting plague, usually because such events dislodge remote wild rodent communities, the natural home of Y. pestis, from their habitats and drive them toward human settlements in search of food and shelter.

Social and demographic conditions are also risk factors in plague. Like other infectious illnesses, the disease requires a minimum population base of four hundred thousand people to sustain itself. When human numbers fall below that base—or people are dispersed too widely—the chain of infection begins to break down. Sanitary conditions are important, too. A principal vector in human plague, the black rat—Rattus rattus—feeds on human refuse and garbage, so the filthier a society’s streets and homes and farms, the larger its plague risk. Since the flea is an even more critical disease vector, personal sanitation matters as well; people who wash rarely are more attractive to an infected flea than those who wash regularly. Humans who live with farm animals are also at greater risk because they are exposed to more rats and fleas; and if a population lives in homes with permeable roofs and walls, the risk is even greater.

The role of malnutrition in human plague is controversial, though perhaps unjustifiably so. It is true that bacteria, which require many of the same nutrients as humans, have more difficulty reproducing in malnourished hosts. But experience with plague in early-twentieth-century China and India suggests that nutritional status, like sanitation, is a risk factor in the disease, and emerging research suggests that nutrition may also affect susceptibility in another, more subtle way. Recent studies have found that exposure to malnutrition in utero damages the developing immune system, creating a lifelong vulnerability to illness in general.

 

* * *

 

From Caffa to the jungles of Vietnam,* war has also been an important predisposing factor in human plague.