What Was the World’s First-Known Pandemic?

The Plague of Justinian in the sixth century was one of the worst of times. “Now the disease spread over the whole world, sparing neither sex nor age nor making any distinction either in respect of rank or of any other quality; for all alike were attacked, whether they lived in luxury or in poverty,” wrote the Byzantine historian Procopius of Caesarea, who lived through it. “It was not possible to calculate the number of the dead, for no one could count them, nor was it possible to bury them all. Many were cast into the sea, and many lay unburied in the streets and in the houses… Whole cities were left desolate, and the plague did not cease until it had destroyed almost all the inhabitants of the world.”

According to Procopius, 10,000 people a day died at Constantinople during the peak of the outbreak in A.D. 542. Modern historians estimate up to a quarter-million people died in the imperial capital, while more than 10 million died throughout the Byzantine Empire. But the Plague of Justinian—named after the emperor Justinian I—did not stop at the empire’s borders. It continued for more than 200 years in other parts of Europe, North Africa and the East, with a global death toll estimated at tens of millions.

The cause of the disease was identified in a 2013 study as the single-celled organism Yersinia pestis, which causes bubonic plague. Researchers found traces of Y. pestis in human skeletons from Bavaria, which verified reports of the symptoms. "When the disease attacked anyone, it was clear to all that it was the plague, for there immediately followed a swelling in the groin or in the armpit or beside the ears or at some other part of the body where there happened to be a gland; and in some cases there appeared black blisters about the size of a lentil," Procopius wrote. Subsequent research has confirmed Y. pestis at other plague sites in the empire, and a July 2025 study found traces of it at Jerash in Jordan, near the empire's heartlands.

Historians have looked to the Plague of Justinian for lessons. According to 20th-century writers, it was "the first pandemic” (in this context Europe’s Black Death was the second pandemic and a plague outbreak in the 19th century was the third).

They noted its lingering effects, such as labor shortages and outbreaks of variant strains of the original disease (the Roman Empire also suffered an Antonine Plague in about 165 and the Plague of Cyprian in about 250, but they are not well documented and the disease that caused them—possibly smallpox—is unknown.)

A study in 2024 suggested the Plague of Justinian was sparked by climate changes, recorded in seafloor sediments, from relatively warm in the imperial heartland to pronounced periods of cold with an average annual temperature several degrees lower than normal. Those climate changes, in turn, seem to have disrupted food supplies and may have increased pests like rats. It was those downstream factors that resulted in the plague outbreak, the researchers suggested.

Tufts University classical historian Brandon McDonald, who studies ancient plagues alongside evidence of climate changes from polar ice cores, says the cold spell that sparked the Plague of Justinian seems to have been caused by two volcanic eruptions—one somewhere in the Northern Hemisphere and another a few years later near the equator—which spread sunlight-reflecting dust and chemicals into the upper atmosphere. "Both were huge, and they could have had devastating effects on the climate," McDonald says.

The effects of the Plague of Justinian were undeniably terrible—especially if one caught the disease, which was often fatal—but it may not have been as bad later as some later writers put it. Oklahoma State University historian Merle Eisenberg cites evidence that the plague was not a civilization-ending event, as some have described it, but that it had "uneven" impact in different regions of the Byzantine Empire.

Eisenberg says Procopius could be forgiven for his gloomy descriptions of the plague because he lived at the heart of the empire in Constantinople—the largest city in the Mediterranean world at the time—where the impact of the disease was much higher. But Eisenberg and his colleague Lee Mordechai have proposed that many historians during and after the Black Death developed a "plague concept" (from their own experiences or records) that assumed the underreported Justinianic Plague was as bad as the much-reported Black Death. But in fact, evidence suggests it was not.

So many millions died during the Black Death, for example, that it resulted in numerous mass graves. But relatively few have been found from the time of the Plague of Justinian, Eisenberg notes. “My perspective on this is that the Justinianic Plague was definitely a significant pandemic that killed millions of people … but what’s really important is its differing impact in different locations,” he says.