A ‘plague’ comes before the fall: lessons from Roman history

The Pax Romana—the 200-year “golden age” of the Roman Empire—was a marvel of diversity, connectivity, and unchallenged hegemony. By the middle of the second century AD, imperial Rome ruled territory across three different continents. Roughly one-quarter of the Earth’s population, some 60 million people, lived under Rome’s vast aegis, and the emperors of the age—most notably Marcus Aurelius—enjoyed the consent of those they governed. The Empire’s elites—witnessing the disciplined legions, widespread religiosity, cultural efflorescence, and dominant economy—likely expected their world order to endure forever.

In the year 166 AD, however, seemingly eternal Rome was caught completely off-guard as a deadly novel disease swept across the Eurasian landmass. It ransacked Rome’s cities for at least a decade and preceded centuries of decline. This major biological event—now known as the Antonine plague—appears to have been the world’s first pandemic.

Historians hotly debate its death toll—with estimates ranging from 2 percent to 35 percent mortality—and its broader social and economic effects. The disease itself remains undiagnosed. The great Greek physician Galen described its main symptoms as fever, throat ulcers, and a pustular rash. Some have suspected it was measles or smallpox, but modern analysis provides reasons to doubt these as the possible culprits. Human remains from the Antonine plague period, meanwhile, have thus far failed to yield genetic evidence sufficient to identify the pathogen.

Although the plague did not on its own cut short Rome’s dominance, it struck an empire that was confronting multiple challenges beneath a veneer of prosperity and growth—factors that modern-day infectious disease experts might recognize as creating the ideal conditions for pandemics. Much remains unknown about the Antonine plague; in some ways, modern scholars are just as in the dark about this first pandemic as its contemporary victims. But interdisciplinary researchers, trying to understand how the plague could have helped push such a powerful empire to the breaking point, have recently been unravelling some of its mysteries.

Probing the plague. Historians, archaeologists, and scientists have been sharing data and expertise, working together to develop histories of past pandemics—including the Antonine plague—that are surprisingly comprehensive and nuanced. Paleogenetic and paleoclimatological evidence reveal the crucial role of environmental and demographic factors in the pandemic. Insights from modern economics and sociology have improved historians’ understanding of how the institutions of the Roman Empire were affected by disease mortality. Even before the pandemic arrived, the pre-existing ecological, economic, and demographic context of mid-second century Eurasia prepared the way for the disease that would accelerate the end of Rome’s era of efflorescence.

Research assessing the severity of modern anthropogenic climate change, for example, has compiled a vast array of climatological data dating back to the Roman period, and well before. Such research offers historians an increasingly detailed and comprehensive view of the ecosystems of ancient Eurasia and Africa. The ancient Mediterranean was (and still is) polka-dotted with microclimates; meanwhile, ice cores from Greenland, ancient tree rings from northern Europe, and sediment cores from Egypt and Italy suggest that some regions in and around Roman territory endured cooler temperatures and droughts about a decade ahead of the Antonine plague pandemic. These climatological shifts were hardly severe, nor did they affect the entire Mediterranean Basin. Many of the affected regions, however, happened to play outsized roles in supplying Roman cities with grain.

The annual Nile flood in Egypt, for instance, reliably nourished well-irrigated grainfields with nutrient-rich water from the Ethiopian highlands. The resulting harvests, often abundant, were stored and then shipped in massive vessels across the Mediterranean to Rome for distribution to the city’s masses. But from the 150s onward, a series of droughts near the Nile headwaters in equatorial east Africa disrupted the flood, reducing the productivity of Rome’s main breadbasket. Meanwhile, at the same time, increased storm activity in the western Mediterranean—as confirmed by sediment cores extracted from the coast of southern France—made shipping already scarce grain far riskier than in previous centuries. As a result, denizens of Rome and several other major cities, and possibly also some of Rome’s soldiers, endured greater food insecurity and malnutrition—weakening their bodies ahead of the pandemic’s arrival in the 160s.

An interconnected, vulnerable ancient world. Historians still don’t know exactly where and when the pandemic entered Roman territory. But, again, historical circumstances conspired in favor of the novel disease.

An outbreak today can jump continents as quickly as an airplane can fly. Travel and transportation can facilitate the spread of infectious diseases. It may not be coincidental, therefore, that by the time of the Antonine plague, the Eurasian landmass was better-connected than ever before. In 166 AD, for the first time in recorded history, the imperial Han court in Luoyang, China, received visitors from the Roman Empire. Merchants from India, sub-Saharan Africa, Arabia, and Egypt rode the trade winds to ports all around the Indian Ocean. Roman soldiers, seeking to police and tax such abundant trade, ventured well outside Roman borders—as Latin inscriptions in the Farasan Islands of southern Arabia attest. In short, there were plenty of opportunities for novel diseases to cross political and geographic barriers into new populations, transforming what might have otherwise been a regional epidemic into a pandemic that spread across three different continents.

In the Roman Empire, an impressive transportation infrastructure—once a source of economic and military power—became a sudden liability once the pandemic breached its borders. Roman roads and ships weren’t themselves responsible, but larger movements and migrations transported the disease from city to city.

Because of the shifts in local climates, and resulting food insecurity, desperate and hungry rural peasants had already flooded into cities in Asia Minor (present day Turkey) and Italy. Beyond Roman borders, nomadic peoples on the Eurasian Steppe in search of food pushed against the Germanic tribes along the Danube River, sending hordes of migrants and invaders into Roman frontier provinces. Contemporary sources from the Han Empire reference a series of epidemics in several Chinese cities, as well as the army. Concerns over ever-present sickness were partly responsible for the famed Yellow Turban Rebellion—a peasant rebellion that unleashed decades of civil war and instability in much of eastern China.

At the exact same time, tens of thousands of Roman soldiers were uprooted from their military bases and sent thousands of miles—first to fight a war on the Empire’s eastern frontier in Persia (Iran), then back into Europe to resist the surging tide of Germanic migrants. At multiple points along these journeys, soldiers could have collected the Antonine plague pathogen.

The plague and the capital. Rome’s large legions might have sustained disease transmission for weeks, if not months, as armies passed back-and-forth through the same densely populated cities of Asia Minor and Italy that were taking on underfed refugees from the budding crisis.

None of these cities, however, were as packed as Rome—a cosmopolis of over one million people. In October of 166 AD, just as the pandemic reached Italy, the city held a massive triumphal parade for the legions, fresh off their victory in Persia. Perhaps 100,000 or more citizens crammed into the city center to celebrate, creating what may have been the world’s first super-spreader event.

Shortly following the triumph, the streets of Rome must have resembled a war zone. Bodies were so strewn about the city that Marcus Aurelius and his co-emperor imposed strict regulations on burials and tombs. They funded corpse removal. They sought out the gods for aid. At some point, perhaps after the first wave abated, the emperors commissioned statues to memorialize elite victims, while the masses were commemorated in remembrance events.

The ancient Romans had limited means to treat the Antonine plague, although they developed many remedies of unknown or suspect effectiveness. Elites, including the emperor Marcus Aurelius, used a concoction called “theriac”—an aged blend of exotic spices and expensive substances, mixed with a dose of opium. Others tried various smell therapies—including smelling laurel leaves. Galen claimed that fresh urine applied directly to the skin could help—the younger the urinator, the better.

The Antonine plague would continue to rage in the cities and military camps of the Roman Empire for at least another decade. A second wave of an undiagnosed epidemic disease hit Rome in 190 AD; if this too was part of the Antonine plague, then the pandemic lasted at least a quarter-century. However long it endured, the plague was an unprecedented test the resilience of Roman systems; Galen named it “the everlasting pestilence.”

Marcus may have rallied Rome during the first wave. But when the plague struck northern Italy a few years later, he abandoned his friends and soldiers to a dark winter of sickness. Officials responded to drought and high grain prices with price controls, most likely disincentivizing production and making shortages even worse. In response to plague and war deaths in the legions, Marcus recruited criminals and slaves into the military. This proved fateful when, a few years later, many of them deserted and, now well-equipped and trained, turned on the cities of the Empire, pillaging and murdering in a crime wave that stretched from Asia Minor to western Europe.

While it might seem like the pandemic single-handedly caused the decline and fall of the Roman Empire, it was clearly more complicated than that. The western Roman Empire would muddle along for over 200 years, but its heyday ended with the Antonine plague. The plague exposed and exacerbated pre-existing fragilities. Many Roman achievements may have been grand, but the Empire was a product of its pre-industrial context, in which weather, famine, and other factors could be destabilizing. The agricultural economy was subject to the vagaries of its ecosystem and the limitations of fledgling markets. Roman cities, for all the attention paid to aqueducts and baths, were contaminated by poor sanitation and grappled with persistent malnutrition. They may have been temporarily well-connected enough to enjoy the commodities of distant regions, but these same populations were simultaneously “immunologically naive” to pathogens from outside their immediate area. While it is no coincidence that the pandemic and the end of the Pax Romana occurred at the same time, exploring the connections between them underscores the interconnectedness and even interdependence of past human societies and their environmental contexts.

Present societies now easily mitigate much of what ailed Rome during the Antonine plague. The wonders of modern medicine—treatments, vaccines, and proven sanitation measures—render once-deadly scourges innocuous or even eradicated. A globalized society is one which collaborates and coordinates—orienting markets, scientific research, and communication channels towards responding to threats and, even better, predicting and preventing them before they occur. And yet, like the Roman Empire, the strengths of the modern world order have inbuilt weaknesses. Travel and transportation are so cheap and easy that pandemic diseases seem virtually impossible to contain.

The collaborative process that permeates most democratic societies nevertheless requires seemingly slow and cumbersome debate and consensus building. Yet all told, the modern world’s capacity to understand and adapt to our natural context—clumsy as it is at times—so far continues to outpace the rapidly evolving diseases that surround us. A vital part of our strategy must be to learn from the pandemics of the past.