Pathogenesis: A History of the World in Eight Plagues

by Jonathan Kennedy

If we trace the Tree of Life to the base of its trunk, we find the Last Universal Common Ancestor (LUCA): a single-celled, bacterium-like organism that is the distant progenitor of all living things, humans included. This one shared antecedent is the reason why everything from blue whales to giant redwoods to bacteria have inherited common features such as DNA to store genetic information and a molecule called ATP that is the universal source of energy. Moving up the tree, the trunk splits into three boughs that represent the great domains of life. Two of these consist of organisms invisible to the naked eye: bacteria and archaea—single-celled microbes that resemble bacteria.[*1] The third branch depicts eukaryotes, which are distinguished by storing their DNA in a nucleus and using specialized structures called mitochondria to produce energy. This category encompasses all complex life including animals, plants and fungi, but it makes up only a few small twigs in the vast Tree of Life. There are about 8.7 million species of animals, plants and fungi on earth,[6] compared to an estimated 1 trillion—1 million million—types of bacteria and archaea.[7] Less than 0.001 percent of all species on the planet are eukaryotes.

It is therefore not the strongest or most intelligent members of our species who were most likely to survive long enough to pass on their DNA to the next generation; rather, it was humans who had the most effective immune system to cope with the onslaught of infectious diseases, or those who had mutations that made their cells unusable to microbes. Lots of these mutations not only conferred resistance to pathogens but also had a negative impact on cell function. This suggests that humans’ struggle for existence was a fight against microbes rather than alpha males and apex predators.

Even the lauded baths were more of a danger than a benefit to public health. In Rome’s bigger institutions thousands upon thousands of people soaked in the same water every day. Bathers didn’t use soap, preferring to cover themselves in olive oil and then scrape it off with an instrument called a strigil. Contemporary writers complained about the water being dirty and contaminated with human excrement. In other words, Roman baths created an ideal environment for waterborne diseases to spread.

Although endemic pathogens killed large numbers of Romans, they also had a surprising benefit: diarrheal disease and malaria created what amounted to a protective force field around the imperial capital. Anyone who had survived until adulthood would have acquired immunity but people who came from outside, including those who wanted to conquer the city, were at high risk of getting sick or dying if they stayed too long. Malaria had prevented attacks on the imperial capital since at least the late third century BCE, when Hannibal managed to cross the Alps with 60,000 troops, 12,000 horses and 37 war elephants. The Carthaginian army routed the Romans on several occasions. It was malaria, which killed Hannibal’s wife, son and many of his soldiers, that brought the invasion to an end and saved Rome.[27] But infectious diseases only worked in Rome’s favor for so long. The pax Romana produced conditions that were ideal for the emergence of epidemic infectious diseases. The growth of long-distance trade with sub-Saharan Africa, India and China increased the risk that Romans would come into contact with novel pathogens—and the well-connected, highly urbanized Roman Empire created a perfect breeding ground for fast-spreading new diseases. It was only a matter of time before an outbreak brought this unprecedented period of peace and prosperity to a sudden halt.

How do we explain the sudden transformation of Christianity from a marginal Jewish sect to a popular religion? The American sociologist Rodney Stark argues that infectious diseases are a crucial part of the story. The Christian faith skyrocketed because it provided a more appealing and assuring guide to life and death than paganism during the devastating pandemics that struck the Roman Empire in the second and third centuries CE. In fact, he goes as far as to say that if it wasn’t for the Antonine and Cyprianic Plagues, “Christianity might never have become so dominant a faith.”[47] One major point of difference between the old and new faith was the issue of what happens when we die. Jesus promised everlasting life in paradise, whereas “pagans believed in an unattractive existence in the underworld.” Then there was the question of why pandemics occur: the Greco-Roman gods were capricious, angry, indifferent to human suffering and sometimes downright cruel. Remember, the Romans believed that Apollo conjured up a plague that killed 7 million people to punish the transgressions of a few light-fingered legionnaires in the middle of the second century. In this belief system, the best way to save oneself from Apollo’s wrath was to try to deflect his anger by making a sacrifice or some other form of offering. In contrast, Jesus’ message that hardship brought redemption was much more reassuring in the face of the recurring devastation of plagues. It offered hope and meaning by exp...

In 1348, Pope Clement VI issued two papal bulls that pointed to fundamental flaws in the well-poisoning libel: Jews were dying in similar numbers to gentiles in continental Europe and the plague was also devastating England, which had expelled its Jewish community in 1290. The Patriarch condemned those taking part in attacks on Jews as having been “seduced by that liar, the devil” and threatened them with excommunication. But his intervention had limited impact. The most powerful secular leader in Europe, Charles IV, the Holy Roman Emperor and King of Bohemia, encouraged the violence.[*1] He offered the Archbishop of Trier the property of those Jews in Alsace “who have already been or may still be killed” and promised the Margrave of Brandenburg the best three houses in Nuremberg “when there is next a massacre of the Jews.”[25] Local political elites took advantage of the situation too. In Cologne, Jewish property was split between the town council and archbishop, and the proceeds were used to beautify the cathedral and build a new town hall. In Augsburg, the man who opened the town gates to assailants from outside—the Bürgermeister, Heinrich Portner—was heavily indebted to Jewish moneylenders.[26] After the massacre, there was no one left alive to collect the money owed.

The death of more than half the population created a crisis for the feudal system. The lords’ economic situation was considerably worse than it had been before the plague: now there were far fewer serfs to provide them with produce and labor. And if the lords had any surplus to sell, the prices they commanded were far lower than before the Black Death because, with fewer mouths to feed, demand had fallen. Consequently, the lords tried to squeeze more out of their serfs in a desperate effort to maintain their livelihoods.[72] At the same time, peasants were keen to take advantage of the shortage of workers and glut of cultivatable land to demand better conditions. The struggle played out in different ways across the continent.[73]