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The Eternal Return of the Plague

The plague conjures up images of the medieval Black Death, and perhaps the vaguely reassuring sense that, in the developed world, such dangers are a scourge of the past. But, as the recent outbreak in Madagascar makes clear, the upper hand humans have over the plague today is tenuous and almost certainly transient.

NORMAN, OKLAHOMA – “Fearsome Plague Epidemic Strikes Madagascar.” That recent New York Times headline might sound like the synopsis of a horror movie. The epidemic gripping Madagascar is not just any plague, and it certainly isn’t some Hollywood apocalypse. It’s the plague, caused by the bacterium Yersinia pestis, agent of the notorious bubonic plague.

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For most people, “the plague” conjures up images of the medieval Black Death, and perhaps a vaguely reassuring sense that, in the developed world, such ancient dangers are long past. But in recent years, thanks to the work of geneticists, archaeologists, and historians, we now know that human civilization and the plague have a much deeper and more intimate association than previously assumed. Lessons learned from studying this historic interaction could reshape how we think about global public health today.

All infectious diseases are caused by pathogens – bacteria, viruses, protozoa, and parasites – that are capable of subverting our immune systems long enough to make us sick. These organisms are the product of their own biological evolution, and the history of the plague’s development is perhaps (along with maybe HIV) the most detailed biography of any pathogen known to science.

The plague bacterium, in its most destructive form, is about 3,000 years old. It evolved in Central Asia as a rodent disease; humans were accidental victims. From the germ’s point of view, people make poor hosts, because we die quickly and are usually a terminus, not a transmitter. The plague is spread principally by the bite of fleas, and a few thousand years ago, the bacterium acquired a genetic mutation that made it ferociously effective at spreading. This adaptation improved the plague’s biological fitness, which, for rodents – and the humans who live near them – has proven to be a nightmare.

Thanks to new genomic evidence, we can say with greater confidence how long this nightmare has been recurring. One of the most surprising and solidly confirmed findings in recent years has been the prevalence of plague in samples from Stone Age and Bronze Age societies in Europe and Central Asia. While it remains unclear what role plague played in the failure of those societies, it is reasonable to assume that the disease has long influenced human history.

What is now beyond question is that Yersinia pestis was indeed the pathogen responsible for two of the most destructive pandemics ever. The Black Death, which lives on in popular imagination to this day, arrived from Central Asia in the 1340s, and in the space of a few years, wiped out roughly half of the population in the regions it struck. The disease then lingered for a few more centuries, killing many more.

But this entire episode is properly known as the “second pandemic.” The first pandemic began in AD 541, during the reign of the Roman Emperor Justinian. The outbreak is known as the Justinianic plague, and, like the Black Death, it cut a swath of destruction from inner Asia to the shores of the Atlantic in the space of a few years. Total mortality was in the tens of millions, and stupefied contemporaries were certain they were living on the verge of the last judgment.

As with the Black Death, later historians questioned whether a rodent disease could cause destruction on such a scale. But in recent years, the pathogen’s genetic traces have been found in sixth-century graves, and the DNA evidence convicts Yersinia pestis of this ancient mass murder as definitively as it would in a modern courtroom. The plague triggered a demographic crisis that helped to topple the Romans’ “eternal empire.”

Plague pandemics were events of mind-boggling ecological intricacy. They involved a minimum of five species, in perilous alignment: the bacterium itself, the reservoir host such as marmots or gerbils, the flea vector, the rodent species in close quarters with humans, and the human victims.

The germ first had to leave its native Central Asia. In the case of the Justinianic plague, it seems to have done so by exploiting the shipping networks in the Indian Ocean. Once within the Roman Empire, it found an environment transformed by human civilization, along with massive colonies of rodents fattened on the ancient world’s ubiquitous granaries. Human expansion helped rodents prosper, and rat infestations, in turn, intensified and prolonged the plague’s outbreak.

There is tantalizing evidence that climate change also played a role in triggering the first pandemic. Just a few years before the appearance of the plague on Roman shores, the planet experienced one of the most abrupt incidents of climate change in the last few thousand years. A spasm of volcanic explosions – in AD 536, when historians reported a year without summer, and again in AD 539-540 – upset the global climate system. The precise mechanisms by which climate events fueled plague remain contested, but the link is unmistakable, and the lesson is worth underscoring: the complex relationship between climate and ecosystems impacts human health in unexpected ways.

The plague in Madagascar today is an offshoot of what is known as the “third plague pandemic,” a global dispersion of Yersinia pestisthat radiated from China in the late nineteenth century. There still is no vaccine; while antibiotics are effective if administered early, the threat of antimicrobial resistance is real.

That may be the deepest lesson from the long history of this scourge. Biological evolution is cunning and dangerous. Small mutations can alter a pathogen’s virulence or its efficiency of transmission, and evolution is relentless. We may have the upper hand over plague today, despite the headlines in East Africa. But our long history with the disease demonstrates that our control over it is tenuous, and likely to be transient – and that threats to public health anywhere are threats to public health everywhere.

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