Chapter 2 – The Antonine Plague and the Roman-Parthian War (165–180 CE)
First draft of Chapter 2 of my War Epidemiology book-in-progress project.
About Sources and Citations
To keep this book accessible and readable, I’ve dispensed with traditional in-text citations and footnotes. All direct quotes, historical details, and interpretations are drawn from published primary and secondary sources, most of which are widely available in multiple editions or can be easily found online. Rather than burden the narrative with academic clutter, I’ve chosen to prioritize clarify.
A complete list of sources referenced in each chapter can be found in the references section at the end of said chapter. Readers seeking deeper explorations are encouraged to consult those texts directly.
While every effort has been made to ensure historical accuracy, this is not a work of scholarly interpretation. It is a work of narrative synthesis, grounded in evidence but written for the curious general reader.
“The ancient world never recovered from the blow inflicted upon it by the plague which visited it in the reign of Marcus Aurelius.”
— Barthold Georg Niebuhr
In 165 CE, Rome stood at its height. The empire stretched from Britain to Syria, from the Rhine to the Sahara. Its roads were smooth, its armies undefeated, its cities humming with trade. At the helm were two co-emperors: Lucius Verus, charismatic and martial, and Marcus Aurelius, the philosopher-king whose stoic calm seemed to promise stability.
Then the soldiers came home.
They returned from the Parthian frontier not just with loot, but with lesions. Some accounts traced it to Seleucia, others to the temples of Mesopotamia, or even farther, such as Ethiopia, Egypt, or the distant east. Wherever it began, by the time the armies reached Rome, the disease was everywhere.
This was not Rome’s first epidemic. But it was the most devastating the empire had yet faced. Tens of millions lived within its borders. And over the next fifteen years, as the Antonine Plague tore through city after city, that number dropped, perhaps by ten million or more. Some estimates suggest the empire lost up to a quarter of its population. No region was untouched. No class was spared.
The Roman Empire didn’t fall from the Antonine Plague. But it put cracks in the foundations of political cohesion, faith in the gods, and confidence in Rome’s divine protection. And in the background was a quiet new reality in the wait: the most powerful military in the world had met an enemy it could not kill.
The Invisible Enemy
When the Roman general Avidius Cassius crossed the Euphrates in 165 CE, he was hunting Parthians, not pathogens. The campaign was meant to strike deep into the heart of Rome’s rival empire and it succeeded beyond expectation. Cities fell. The Parthians retreated. Roman standards were planted along the Tigris. But something else followed the legions home. As soldiers returned from the East, they brought back more than loot. They brought disease.
The Antonine Plague, or Plague of Galen, named for the physician and philosopher Aelius Galenus, swept through the empire with stunning speed. Rome’s armies, trade networks, and population density created the ideal conditions for transmission. At its peak, the epidemic killed as many as 2,000 people per day in the capital (Cassius Dio, Roman History, LXXII.14). The philosopher-emperor Marcus Aurelius, battered by war and personal illness, compared the spread of disease to a rot of the soul. Others were less philosophical. They fled. They panicked. They buried their dead by the wagonload.
Modern scholars believe the Antonine Plague may have been smallpox, its symptoms recorded by the physician Galen during outbreaks in Rome and Aquileia. But there is no physical confirmation, and other diagnoses persist. What is certain is the scale. Between 165 and 180 CE, the Antonine Plague crippled the Roman army, sapped its economy, and forced the empire to recruit slaves and gladiators to keep fighting. The Roman world did not collapse. But it flinched, and that tremor carried into the centuries that followed.
War and Pathogens: The Parthian Campaign
The Antonine Plague began with war. In 161 CE, co-emperors Marcus Aurelius and Lucius Verus responded to a crisis in the East: Vologases IV of Parthia had invaded Armenia, deposed Rome’s client king, and seized territory in Cappadocia and Syria (Birley, 2001, p. 121). Lucius Verus, more soldier than scholar, took command of the counteroffensive.
Roman forces, led in part by General Avidius Cassius, advanced deep into Parthian territory. They recaptured Armenia, seized Nisibis, and eventually took the Parthian capital of Ctesiphon. Some sources claim that the neighboring city of Seleucia was also sacked, possibly in violation of a surrender (Cassius Dio, Roman History, LXXI.2; Birley, 2001, pp. 140–141). It was during or shortly after this campaign that the plague first appears in the historical record.
Lucian, in a mocking account, reports a mythic origin: a soldier accidentally opened a cursed casket in a Seleucian temple, unleashing a "pestilential vapor" (Lucian, Alexander the False Prophet). While this is surely apocryphal, its narrative power reflects a deeper truth. People believed the Romans had disturbed something ancient and unclean. And in epidemiological terms, it may not be far off. Urban warfare, refugee movements, and exposure to new environments could have easily sparked a zoonotic spillover or accelerated the spread of a pre-existing regional epidemic (Thacker, 2020, pp. 17–18).
After the eastern campaigns, returning legions became the unwitting vectors of a continent-wide catastrophe. The disease spread along Roman roads, ports, and fortresses from Syria to Gaul, from North Africa to Britain. Wherever Roman soldiers marched, the plague followed.
The Plague Hits Rome
By 166 CE, the epidemic reached the capital. Galen, the empire’s most renowned physician, documented cases in both Rome and Aquileia, the northern city where Marcus and Verus later set up their military headquarters. Galen described a distinctive clinical picture: fever, black diarrhea, skin eruptions (sometimes pustular), and ulcers leading to scabbing (Littman & Littman, 1973, pp. 246–248).
These symptoms match several candidates, including typhus, measles, and even hemorrhagic plague, but the most widely accepted diagnosis is smallpox. In particular, its hemorrhagic variant (Cunha & Cunha, 2008, pp. 10–12). Galen’s writings are not systematic case studies; his interest lay more in treatment than taxonomy. But his observations remain the best medical record we have.
Death tolls are speculative, but Cassius Dio estimated 2,000 deaths per day in Rome at the height of the outbreak (Roman History, LXXII.14). Elsewhere, mass graves in Gloucester (Glevum) and Tomis suggest widespread mortality as far as Britain and the Balkans (Thacker, 2020, p. 24).
The Roman state responded with public funeral rites, legal reforms to regulate burial, and extraordinary economic measures. Marcus Aurelius auctioned imperial possessions to fund the war effort and public aid (Birley, 2001, p. 160). Laws were passed to prevent the illegal sale of burial plots and restrict corpse transport through urban areas.
Even animals were not spared. In Smyrna, the orator Aelius Aristides recorded that his household and livestock succumbed to the plague. He barely survived (Aristides, Orations XLVIII, 38–45).
The imperial court did not escape untouched. Lucius Verus died in 169 CE likely from stroke, though disease remains a possibility (Birley, 2001, pp. 156–158). Marcus Aurelius, always prone to illness, would later instruct his son Commodus to keep away from his deathbed in case the disease remained contagious (Magie, Scriptores Historiae Augustae, Marcus Aurelius Antoninus, 28).
But the plague did not just devastate Rome. It reshaped its wars. And those, in turn, reshaped history.
War on Two Fronts: The Marcomannic Campaigns and Rome in Crisis
By the late 160s CE, the Antonine Plague was no longer just an eastern affliction. It had reached the city of Rome and the frontiers of the empire, spreading along the well-worn paths of commerce and conquest. Soldiers, having returned from the Parthian campaigns, brought with them not only spoils but also sickness. The city’s famed infrastructure or roads, aqueducts, and interconnected provinces now served as vectors of contagion. And just as the empire was beginning to reckon with this invisible enemy, a very visible one arrived on its northern borders.
In 167 CE, the tribes of the Danube frontier, including the Marcomanni, the Quadi, and others, launched a series of raids into Roman territory. These incursions came just as the Parthian War was winding down and at a time when the Roman military had been badly weakened by plague. The Langobardi and Obii crossed the Danube into Pannonia, prompting a swift counteroffensive. Yet this first wave was only the beginning of what would become the Marcomannic Wars (Birley, p. 149).
Rome’s military predicament was dire. Orosius, writing in the fifth century, noted that the plague had decimated the legions: “The Roman army and all its legions stationed far and wide in their winter quarters lost so many men that the Marcomannic war… could not have been waged without the fresh levy of troops” (Orosius, VII.15). Recruiting replacements took years. Marcus Aurelius was forced to take extraordinary measures, including enlisting slaves, hiring gladiators, even auctioning off palace treasures to raise funds for the military effort (Birley, pp. 159–160).
Despite these setbacks, Aurelius pushed forward. He knew that the integrity of Rome’s northern frontier depended on projecting strength. In 168 CE, the co-emperors Marcus and Lucius Verus established a military headquarters in Aquileia to oversee the campaign (Thacker, p. 26). Galen, who had returned to Rome from the front lines earlier, would later describe the intense mortality that occurred during this time, likely referencing Aquileia itself. The prefect Furius Victorinus died along with a large portion of his forces. While some sources attribute these deaths to battle, others strongly suggest the Antonine Plague as a more probable cause (Birley, p. 155).
When Lucius Verus died of a sudden illness, possibly a stroke, Marcus was left to lead the war alone (Birley, pp. 156–158). His return to Rome was not just a political move; it was also a practical one. The state’s finances were strained, and the manpower crisis demanded immediate solutions. He enacted a policy of freeing slaves in exchange for military service and filled the ranks with provincials and prisoners of war.
And still, the war dragged on.
Rome’s enemies had grown bolder. In 170 CE, barbarian forces crossed the Alps, a feat not accomplished since the Cimbri and Teutones over two centuries earlier, and sacked several cities in northern Italy, including the ancient town of Opitergium (Birley, p. 165). Even Aquileia, Rome’s military hub for the region, came under siege. According to Cassius Dio, twenty thousand Roman soldiers were killed during these invasions (Dio, Book LXXI.3).
Yet the empire held. Marcus Aurelius launched a counteroffensive and began to push the Marcomanni and their allies back across the Danube. Between 171 and 175 CE, Rome retook lost territory and exacted harsh terms from the tribes. Some surrendered captives and livestock; others offered troops for conscription. Aurelius even entertained the idea of turning conquered territory north of the Danube into a new province, a plan cut short by a rebellion in the East (Thacker, p. 28).
What the Antonine Plague had started in a slow hemorrhaging of Rome’s population, army, and treasury, the war threatened to finish. But under Aurelius’s leadership, the empire did not collapse. It adapted. Rome replenished its army with foreign recruits, secured its borders through negotiated settlements, and retained control of key territories. Yet these adaptations came at a cost. The empire’s composition began to shift. Military service became more diverse, internal unity more fragile, and faith in the old gods more precarious.
The war against the Marcomanni didn’t end with a triumphant parade. It ended with a tired empire, a tired emperor, and an uneasy peace. But it also marked a turning point, not just in military history, but in the history of pandemics and power. Rome had survived its twin crises. But it would never be the same.
Symptoms, Scars, and Speculation
The Antonine Plague left few direct traces. No viral fragments recovered from skeletons. No preserved tissue samples. No smoking gun in the archaeological record. What we have instead are words, mostly from a single man.
Galen of Pergamon, Rome’s most famous physician, observed the epidemic firsthand. He treated patients in Rome and again during a later outbreak in Aquileia. Though Galen’s primary concern was treatment rather than description, his writings offer a rare glimpse into the symptomatology of the disease. He mentioned persistent fever, blackened diarrhea, nausea, vomiting, bad breath, and, in the most distinctive detail, a pustular rash that sometimes ulcerated and scabbed over (Littman & Littman, 1973, pp. 246–248). The scabs reportedly covered the body and, in some cases, were preceded by skin discoloration or dark spots, possibly hemorrhaging. Galen also noted cases of internal ulcers and intestinal bleeding, particularly when patients presented with gastrointestinal symptoms first.
These observations provide fertile ground for retrospective diagnosis. But they also present problems. Galen did not write with modern clinical categories in mind. He emphasized symptoms that indicated treatment response, not those useful for etiological identification. Some symptoms, like fever, diarrhea, and cough are so common across infections that they help little. Others, like the pustular rash, offer more specific clues.
Of all the diseases proposed, smallpox remains the most convincing. The presence of a pustular rash that ulcerates and scabs, along with black diarrhea and internal bleeding, aligns closely with the hemorrhagic form of smallpox, a severe and often fatal variant. Littman and Littman argued that this form of the disease best explains Galen’s observations, particularly the high fatality rate and internal symptoms (Littman & Littman, 1973, pp. 248–252). Supporting this diagnosis is the long duration of the epidemic and its return in multiple waves, patterns consistent with a highly contagious, immunizing virus introduced to a largely naive population.
Measles was long considered a contender, especially given its later historic impact in unexposed populations. But genetic and evolutionary studies cast doubt on this timeline. A 2010 phylogenetic study places the divergence of measles from its closest viral relative, rinderpest in cattle, in the 11th or 12th century CE (Furuse et al., 2010). That timing makes measles biologically implausible for a second-century pandemic. Additionally, the Antonine Plague’s case fatality rate appears to have been far higher than that typical of measles, even in virgin-soil populations.
Typhus has also been proposed, especially given the epidemic’s reach through military encampments and cities with poor sanitation. Typhus, spread by lice, causes fever, rash, delirium, and often gangrene in advanced cases. But Galen did not describe the central symptom of epidemic typhus, a flat red rash spreading from the trunk to the limbs. Nor did he mention the louse-borne transmission pathway, which would have influenced Hippocratic-era medical theories of “bad air” or humoral imbalance.
Some scholars have considered even more exotic culprits. Castiglioni suggested bubonic plague or typhus, though this has not gained much traction due to the absence of swollen lymph nodes (buboes) in Galen’s accounts (Castiglioni, 1975, p. 244). Others have speculated about hemorrhagic fevers similar to Ebola, or zoonotic viruses introduced via trade or conquest, particularly in light of the plague’s likely eastern origin (Thacker, 2020, p. 18). But without genetic or molecular evidence, such theories remain in the realm of historical speculation.
Perhaps the most realistic explanation is a syndemic, a co-occurrence of several diseases exploiting the same ecological niche. If smallpox struck first, it could have paved the way for opportunistic bacterial infections such as typhoid or dysentery, especially in malnourished or immunocompromised hosts. The military and urban populations were crowded, stressed, and mobile which offered ideal conditions for multiple pathogens to circulate. Galen’s inconsistent symptoms may reflect this complexity more than the signature of any single virus.
What remains undisputed is the novelty of the disease to Roman society. Whether smallpox or another pathogen, the Antonine Plague behaved like a classic virgin-soil epidemic. It struck a population with no existing immunity, spread rapidly through connected settlements, and killed indiscriminately. Galen remarked that those who recovered did not become sick again, a detail that only strengthened modern suspicions of smallpox, which confers lifelong immunity after survival (Littman & Littman, 1973, p. 250).
That detail also raises a chilling thought. The very concept of immunity was foreign to Roman medicine. The Hippocratic tradition emphasized imbalances of heat and cold, wetness and dryness, not immune memory. Galen may not have understood why survivors were spared in later waves, but he noticed the pattern. In a medical system that lacked the vocabulary of pathogens and antibodies, observation remained the first tool of science.
And so, in a world before microscopes, before inoculation, before even the most basic concept of viral disease, a physician wrote down what he saw. His patients died in front of him. Their skin broke out in blisters. Their bowels emptied black. Their families vanished. The city of Rome, with its temples, arches, and emperors, became a city of carts filled with corpses.
The Antonine Plague did more than kill. It challenged what it meant to be Roman and what it meant to be safe within the bounds of an empire said to stretch to the edges of the earth. The legions could hold the line. But not against fever.
An Empire Transformed
The Antonine Plague did not topple Rome. But it transformed it.
By the time the worst had passed in 180 CE, the death toll had carved its way through every rank of society, from provincial farmers to urban elites, from laborers to senators, from soldiers to emperors. Cassius Dio wrote that at the peak of the outbreak, Rome lost 2,000 people per day (Dio, Roman History, LXXII.14). While these numbers are debated, the broader demographic impact is not: the empire was depleted.
Recruitment into the legions faltered. Marcus Aurelius resorted to freeing slaves in exchange for military service, a controversial break from tradition. Gladiators, provincials, and even criminals were conscripted to fill the gaps (Birley, 2001, pp. 159–160). The Antonine Plague became not just a health crisis, but a manpower crisis. By 170 CE, even the most routine imperial tasks like tax collection and grain distribution faltered in some provinces. The imperial bureaucracy did not collapse, but it slowed under the weight of attrition.
The response from Rome’s political class ranged from stoic endurance to theatrical desperation. Marcus Aurelius auctioned off imperial treasures in the Forum of Trajan to fund the war effort and public relief (Birley, 2001, p. 160). He sponsored statues to honor those lost, ordered public burials for the poor, and tried to project calm in the face of horror. But beneath these efforts, the social fabric was fraying.
Funerary regulations were revised to prevent corpse transport through cities and restrict burial near homes (Birley, 2001, pp. 150–151). These were not just sanitary laws. They were acts of public reassurance, mere attempts to reassert control over death in a time when death was uncontrollable.
And amid the uncertainty, familiar patterns emerged: fear demanded targets. Christians, already seen as outsiders and stubborn in their rejection of the Roman pantheon, became scapegoats. Rumors spread that their impiety had offended the gods. Sporadic persecutions increased under Marcus and Commodus, often linked to accusations of magical thinking, ritual denial, or divine insult (McLynn, 2009, p. 462). Though Christianity would later become the empire’s official religion, in this moment it served as a convenient outlet for civic panic.
The plague also changed the military geography of the empire. To repopulate border garrisons and maintain control, Marcus allowed some Germanic tribes, formerly enemies of the state, to settle inside Roman territory in exchange for service and peace (Cassius Dio, Roman History, LXXI.11–12). This pragmatic decision, born of necessity, laid early groundwork for a recurring pattern: integration of outsiders into the heart of the Roman system. It was not collapse, but it was a shift. The homogeneity of the early empire was giving way to something looser, more federated, more fragile.
Even imperial succession bore the mark of the plague. Marcus Aurelius, before his death in 180 CE, kept his son Commodus away from his sickbed, fearing contagion (Magie, Scriptores Historiae Augustae, Marcus Aurelius Antoninus, 28). The gesture was both fatherly and fatalistic. He knew he was dying. He knew the disease had no cure. Commodus would inherit the empire, not at its height, but in the long shadow of exhaustion.
The death of Marcus marked more than the end of a reign. Edward Gibbon famously called it the close of the empire’s “happiest age.” And though Gibbon’s nostalgia for stoic emperors may be romanticized, the sentiment contains a kernel of truth. After Marcus, Rome became more erratic. Commodus would rule poorly. Civil wars would return. And plagues, though none as severe as this one, would continue to strike, each time leaving deeper wounds.
The Antonine Plague exposed the limits of imperial strength. It did not destroy the empire. But it left it leaner, harder, and more brittle. The legions still marched. The grain still flowed. The gods were still honored. But something had changed. The illusion of control was gone.
Disease had done what no enemy army could do. It entered the bloodstream of empire, unchallenged and unseen. It struck not at the frontiers, but at the heart. And it left a scar that never fully healed.
References
1. Aristides, Aelius. The Complete Works: Volume II, Orations XVII–LIII. Translated by Charles A. Behr. Leiden: E. J. Brill, 1981.
2. Birley, Anthony. Marcus Aurelius: A Biography. London: Routledge, 2001.
3. Cassius Dio. Roman History. Translated by Earnest Cary. Loeb Classical Library, Vols. 8–9. Cambridge, MA: Harvard University Press, 1927.
4. Cunha, Cheston B., and Burke A. Cunha. “Great Plagues of the Past and Remaining Questions.” In Paleomicrobiology: Past Human Infections, edited by Didier Raoult and Michel Drancourt, 1–16. Berlin: Springer-Verlag, 2008.
5. Furuse, Yuki, Akira Suzuki, and Hitoshi Oshitani. “Origin of Measles Virus: Divergence from Rinderpest Virus Between the 11th and 12th Centuries.” Virology Journal 7 (2010): 52.
6. Littman, Robert J., and M. L. Littman. “Galen and the Antonine Plague.” The American Journal of Philology 94, no. 3 (1973): 243–255.
7. Lucian. The Works of Lucian of Samosata. Translated by H. W. Fowler and F. G. Fowler. Oxford: Clarendon Press, 1905.
8. Magie, David, trans. Scriptores Historiae Augustae: Marcus Aurelius Antoninus. Loeb Classical Library. Cambridge, MA: Harvard University Press, 1921.
9. McLynn, Frank. Marcus Aurelius: A Life. Cambridge, MA: Da Capo Press, 2009.
10. Orosius, Paulus. Seven Books of History Against the Pagans: The Apology of Paulus Orosius. Translated by Irving Woodworth Raymond. New York: Columbia University Press, 1936.
11. Thacker, Brenda. “The Antonine Plague: Unknown Death Within the Roman Empire, 165–180 CE.” In Epidemics and War: The Impact of Disease on Major Conflicts in History, edited by Rebecca M. Seaman, 17–31. Santa Barbara, CA: ABC-CLIO, 2020.