The threat of pandemic disease

Bill Gates is worried. The tech entrepreneur and philanthropist has beenusing his megaphone to warn us of the catastrophic risk posed byinfectious diseases. In the Western world, where mortality from lethalgerms has mostly receded into the background, the burden of infectiousdisease can seem like someone else’s problem. But the struggle betweenhumanity and infectious disease is never someone else’s problem –certainly not in our globally interconnected society. And while modernmedicine has the upper hand on many old microbial enemies, we shouldbeware the sinister ability of pathogen evolution to thwart ourcleverest weapons.Gates is right. The risk of a “big one,” a biological event thatthreatens to break down our public health infrastructure and rattle thefoundations of the global order, is out there, lurking. On his side,Gates has legions of epidemiologists whose dire assessments can seemabstract. Human history offers us a deeper and more tangible sense ofthe unpredictable role that invisible biological enemies have played inthe story of our species.Take the example of Rome. By any estimate, the Romans built one ofhistory’s most extraordinary civilisations. For hundreds of years, theRoman Empire controlled territory stretching from the frostbit frontiersof northern Britain to the scorching edges of the Sahara. The capacityto integrate conquered societies into the empire was a source ofstrength and staying power. Roman civilisation was a lunge towardmodernity, with greater social complexity and economic prosperity thanever seen before. And the fall of the Roman Empire represented thesingle greatest step backward in the long but uneven march of humancivilisation.What accounts for this epochal setback? There has never been a shortageof answers: Loss of virtue (an old favourite, but long out of fashion),class conflict, fiscal unsustainability, the technological developmentof “barbarian” civilisations, or (alas) some failure of immigrationpolicy. Some 200 answers have been compassed, and the vast majority ofthem are human – all too human.Historians inevitably make the best use of the evidence at theirdisposal, and in the last few years, that evidence has beenrevolutionised. What we are learning, principally from pathogengenomics, is that the fall of the Roman Empire may have been abiological phenomenon.The most devastating enemy the Romans ever faced was Yersinia pestis,the bacterium that causes bubonic plague and that has been the agent ofthree historic pandemics, including the medieval Black Death. The firstpandemic interrupted a remarkable renaissance of Roman power under theenergetic leadership of the emperor Justinian. In the course of threeyears, this disease snaked its way across the empire and carried offperhaps 30mn souls. The career of the disease in the capital is vividlydescribed by contemporaries, who believed they were witnessing theapocalyptic “wine-press of God’s wrath,” in the form of the hugemilitary towers filled with piles of purulent corpses. The Romanrenaissance was stopped dead in its tracks; state failure and economicstagnation ensued, from which the Romans never recovered.Recently the actual DNA of Yersinia pestis has been recovered frommultiple victims of the Roman pandemic. And the lessons are profound.In the first place, the biological agent of the great plague was arelatively young species. Y. pestis was not a germ that had existed forhundreds 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 arriveon the scene, the results can be tremendously destabilising – a reminderto modern societies that we must do more than keep track of knownthreats.Second, the Roman pandemic was no parochial affair. The closest knownrelatives of the strain that caused the Roman outbreak have been foundin western China. This fact is consistent with the detail provided byancient sources that the pandemic erupted on the coast of Egypt, at anentrepot of the bustling Red Sea trade. The deadly package was ferriedinto the empire across the vast Indian Ocean trade network that broughtsilk and spices to Roman shores. The plague was, then, an unintendedside effect of incipient globalisation.And finally, it was an event of mind-boggling ecological complexity.Plague is a disease of rodents, and the Roman pandemic event involved atleast five different species: the bacterium, the rodents of centralAsia that were the reservoir host, the Black rats that carried the germto the west, the fleas whose bite transmits the disease between hosts,and the human victims. The plague was, in short, a conspiracy of humancivilisation and nature, in a way that the Romans could not haveforeseen or imagined.Rome was far from the only advanced society shaken to its core by theexplosive force of infectious diseases. The medieval Black Death sentsome leading-edge polities (like the communities of Italy) backward,while opening the space for the ascent of others, such as England. Thelethal role of pathogen exchange in the European conquest of the NewWorld is relatively famous, if still imperfectly understood. Thelightning dispersal of cholera around the globe in the 1810s, and the1918 Spanish flu, caused by H1N1 influenza virus, are further examplesof the devastation that germs can unleash when human societies offerthem the right conditions.We are not as helpless in the face of infectious disease as pastsocieties. We have germ theory and public health and antibioticpharmaceuticals at our disposal. But the patterns of history can deepenour sense of the laws that govern civilisation. Often, those laws arenature’s laws, not humanity’s. Evolution is the great wild card, and itsawesome power can be checked but never fully conquered.The threat of pandemic disease deserves to rank among our most rationalfears. Perhaps the experience of bygone civilisations can make thatwarning a little less abstract.* Kyle Harper is senior vice-president and provost and professor ofClassics and Letters at the University of Oklahoma, and the author ofthe new book The Fate of Rome: Climate, Disease, and the End of anEmpire.