Bill Gates is worried. The tech entrepreneur and philanthropist has been using his megaphone to warn us of the catastrophic risk posed by infectious diseases. In the Western world, where mortality from lethal germs has mostly receded into the background, the burden of infectious disease can seem like someone else's problem. But the struggle between humanity and infectious disease is never someone else's problem — certainly not in our globally interconnected society. And while modern medicine has the upper hand on many old microbial enemies, we should beware the sinister ability of pathogen evolution to thwart our cleverest weapons.
Gates is right. The risk of a "big one," a biological event that threatens to break down our public health infrastructure and rattle the foundations of the global order, is out there, lurking. On his side, Gates has legions of epidemiologists whose dire assessments can seem abstract. Human history offers us a deeper and more tangible sense of the unpredictable role that invisible biological enemies have played in the story of our species.
Take the example of Rome. By any estimate, the Romans built one of history's most extraordinary civilizations. For hundreds of years, the Roman Empire controlled territory stretching from the frostbit frontiers of northern Britain to the scorching edges of the Sahara. The capacity to integrate conquered societies into the empire was a source of strength and staying power. Roman civilization was a lunge toward modernity, with greater social complexity and economic prosperity than ever seen before. And the fall of the Roman Empire represented the single greatest step backward in the long but uneven march of human civilization.
What accounts for this epochal setback? There has never been a shortage of answers: Loss of virtue (an old favorite, but long out of fashion), class conflict, fiscal unsustainability, the technological development of "barbarian" civilizations, or (alas) some failure of immigration policy. Some 200 answers have been compassed, and the vast majority of them are human — all too human.
Historians inevitably make the best use of the evidence at their disposal, and in the last few years, that evidence has been revolutionized. What we are learning, principally from pathogen genomics, is that the fall of the Roman Empire may have been a biological phenomenon.
The most devastating enemy the Romans ever faced was Yersinia pestis, the bacterium that causes bubonic plague and that has been the agent of three historic pandemics, including the medieval Black Death. The first pandemic interrupted a remarkable renaissance of Roman power under the energetic leadership of the emperor Justinian. In the course of three years, this disease snaked its way across the empire and carried off perhaps 30 million souls. The career of the disease in the capital is vividly described by contemporaries, who believed they were witnessing the apocalyptic "wine-press of God's wrath," in the form of the huge military towers filled with piles of purulent corpses. The Roman renaissance was stopped dead in its tracks; state failure and economic stagnation ensued, from which the Romans never recovered.
Recently the actual DNA of Yersinia pestis has been recovered from multiple victims of the Roman pandemic. And the lessons are profound.
In the first place, the biological agent of the great plague was a relatively young species. Y. pestis was not a germ that had existed for hundreds of thousands of years. To use our contemporary terminology, when it struck the Roman Empire it was an "emerging infectious disease." As old germs evolve new molecular tools, or entirely new germs arrive on the scene, the results can be tremendously destabilizing — a reminder to modern societies that we must do more than keep track of known threats.
Second, the Roman pandemic was no parochial affair. The closest known relatives of the strain that caused the Roman outbreak have been found in western China. This fact is consistent with the detail provided by ancient sources that the pandemic erupted on the coast of Egypt, at an entrepôt of the bustling Red Sea trade. The deadly package was ferried into the empire across the vast Indian Ocean trade network that brought silk and spices to Roman shores. The plague was, then, an unintended side effect of incipient globalization.
And finally, it was an event of mind-boggling ecological complexity. Plague is a disease of rodents, and the Roman pandemic event involved at least five different species: the bacterium, the rodents of central Asia that were the reservoir host, the Black rats that carried the germ to the west, the fleas whose bite transmits the disease between hosts, and the human victims. The plague was, in short, a conspiracy of human civilization and nature, in a way that the Romans could not have foreseen or imagined.
Rome was far from the only advanced society shaken to its core by the explosive force of infectious diseases. The medieval Black Death sent some leading-edge polities (like the communities of Italy) backward, while opening the space for the ascent of others, such as England. The lethal role of pathogen exchange in the European conquest of the New World is relatively famous, if still imperfectly understood. The lightning dispersal of cholera around the globe in the 1810s, and the 1918 Spanish flu, caused by H1N1 influenza virus, are further examples of the devastation that germs can unleash when human societies offer them the right conditions.
We are not as helpless in the face of infectious disease as past societies. We have germ theory and public health and antibiotic pharmaceuticals at our disposal. But the patterns of history can deepen our sense of the laws that govern civilization. Often, those laws are nature's laws, not humanity's. Evolution is the great wild card, and its awesome power can be checked but never fully conquered.
The threat of pandemic disease deserves to rank among our most rational fears. Perhaps the experience of bygone civilizations can make that warning a little less abstract.
Kyle Harper is senior vice president and provost and professor of Classics and Letters at the University of Oklahoma, and the author of the new book "The Fate of Rome: Climate, Disease, and the End of an Empire."