War creates human remains and refuse, which attract rats; filthy bodies, which attract fleas; and stresses, which can lower immune system function. Marching soldiers and cavalry also help to make a pestilence more mobile.

The historical evidence suggests that the existence of only a few of these conditions is not enough to ignite a pandemic, or major outbreak of plague. The Victorian West, for example, was far more densely interconnected and populated than medieval Europe, but when a major wave of plague swept through China and India a century ago, relatively healthy populations, relatively good sanitation and public health standards, and a sturdy physical plant—wood and brick houses—prevented the plague from gaining a toehold in either America or Europe. The disease reached the West, but after causing a few hundred deaths in Oakland, San Francisco, Glasgow, Hamburg, and several other cities, it died out.

The era that was once called the Dark Ages and is now referred to (less judgmentally, if no more accurately) as the Early Middle Ages also had several conditions associated with plague, including widespread violence, disorder, malnutrition, and filth—if early medieval Europeans washed or changed their clothes more than once or twice a year, it was the best-kept secret in Christendom. However, foreign trade had virtually disappeared, and the rise of hostile new Muslim states in the Middle and Near East put the plague foci of Central Asia and Africa at a further remove from Europe. Also, the early Middle Ages was a period of profound depopulation. In the sixth and seventh centuries a crumbling Roman Europe lost as much as one-half to two-thirds of its population. From Scotland to Poland, the inheritors of a once great civilization lived huddled together like fugitives in forest clearings. Even if, by some fluke, Y. pestis had managed to travel to the early medieval West, it would have failed as miserably as it did in the streets of Victorian San Francisco.

 

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By contrast, the environment of the fourteenth century was well suited to Y. pestis. By modern standards, the population of medieval Europe was relatively low: about 75 million, compared to today’s nearly 400 million. However, compared to the resources available to the population, the continent had become dangerously overcrowded. The years between 1000 and 1250 were a period of great economic and demographic growth in the medieval West, but when the economy began to stall out after 1250, Europe found itself trapped in what historian David Herlihy has called “Malthusian deadlock.” Medieval Europeans were still able to feed, clothe, and house themselves, but because the balance between people and resources had become very tight, just barely. A worsening climate made the margin between life and death even narrower for tens of millions of Europeans. Between 1315 and 1322 the continent was lashed by waves of torrential rain, and by the time the sun came out again in some places 10 to 15 percent of the population had died of starvation. In Italy especially, malnutrition remained widespread and chronic, right until the eve of the plague.

In the fourteenth century war was almost as much a constant as hunger. Italy, where the papacy and Holy Roman Empire were fighting for ascendancy, had descended into a Hobbesian state of all against all. There were large, small, and in-between-sized wars raging in the papal states around Orvieto, Naples, and Rome. At sea, Italy’s “two torches,” as Petrarch called Genoa and Venice, were locked in an interminable maritime conflict. And almost everywhere up and down the peninsula, roving bands of condottieri (mercenaries) were waging fierce little freelance wars. To the north and west, conflict was raging in Scotland, Brittany, Burgundy, Spain, and Germany, and in the ports and plains and cities of northern France the English and French were fighting the first battles of the Hundred Years’ War.

“The city makes men free,” medieval Germans told one another, but a combination of people, rats, flies, waste, and garbage concentrated inside a few square miles of town wall also made the medieval city a human cesspool. By the early fourteenth century so much filth had collected inside urban Europe that French and Italian cities were naming streets after human waste. In medieval Paris, several street names were inspired by merde, the French word for “shit.” There were rue Merdeux, rue Merdelet, rue Merdusson, rue des Merdons, and rue Merdiere—as well as a rue du Pipi. Other Parisian streets took their names from the animals slaughtered on them. There was a Champs-Dolet—roughly translated as “field of suffering and cries”—and l’Echorcheire: “place of flailing.” Every town of any size in Europe had its equivalent of l’Echorcheire: an outdoor slaughterhouse, where butchers in bloodstained clothing cut and chopped and sawed amid discarded body parts and offal and the agonizing moans of dying animals. One irate Londoner complained that the runoff from the local slaughterhouse had made his garden “stinking and putrid,” while another charged that the blood from slain animals flooded nearby streets and lanes, “making a foul corruption and abominable sight to all dwelling near.” In much of medieval Europe, sanitation legislation consisted of an ordinance requiring homeowners to shout, “Look out below!” three times before dumping a full chamber pot into the street.

The medieval countryside, where 90 percent of the population lived, was an even more dangerous place than the medieval city. Thinly walled, peasant homes were highly permeable, and the rat-to-person ratio tended to be very high in rural areas. Urban rat colonies usually divided their attention among several homes on a street, but in the country, not uncommonly, a single peasant family would find itself the target of an entire rodent colony.

The medieval body was in as shocking a state as the medieval street. Edward III scandalized London when he bathed three times in as many months.