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 civilisations. 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 civilisation 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
civilisation.
What accounts for this epochal setback? There has never been a shortage
of answers: Loss of virtue (an old favourite, but long out of fashion),
class conflict, fiscal unsustainability, the technological development
of “barbarian” civilisations, 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
revolutionised. 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 30mn 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 destabilising – 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
entrepot 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 globalisation.
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
civilisation 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 civilisation. 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 civilisations 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.