How Did the Plague of Justinian Change History?
The Plague of Justinian (541–542 CE) was one of the worst plagues in recorded history, arguably bringing two major empires to devastation and affecting numerous societies across Eurasia. The only other known event comparable to its impact was the Black Death of the 14th century. These two plagues are perhaps even related, as both seem to start in Central Asia and are based on the plague carried by rats.
The Plague of Justinian was Similiar to Bubonic Plague
To our knowledge, the bacterium that caused the Justinian Plague (Figure 1) Yersinia pestis, can still be found in the mountains of Tian Shan. The mountains sit along the modern Kyrgyzstan, Kazakhstan, and China borders. This bacteria was a form of bubonic plague, very similar to the Black Death. While the disease from this bacteria can be easily treated with anti-biotics today, periodic outbreaks would have likely devastated ancient populations without any known immunity.
The plague could have spread to the Byzantine Empire for many reasons. Still, it could have been migrating travels from Central Asia, including possibly Huns migrating towards Europe around this time, helping to spread the plague. This era was also during a period of relatively cold winters and failed crops that may have prompted migrations that brought the disease with travelers.
The Silk Road was also an extensive trading network during the 6th century C.E., suggesting travelers from Central Asia probably brought it to the eastern Mediterranean, where it then also spread into Europe from these Mediterranean ports. There is also evidence that the plague came through North Africa, as important trading ports connected Africa with eastern Asia. From Ethiopia, and into Egypt, the plague could have also expanded into the Middle East and Europe.
Both historical descriptions of the symptoms and excavated skeletons with evidence of the plague that date to this period suggest the plague, similar to the 14th century Black Death, was the culprit.
The Plague spread from Central Asia into Northern Europe
Procopius was a Byzantine historian, and he reported an outbreak of the plague in 541 in the Egyptian port of Pelusium. This report might support the idea the disease entered Europe through North Africa, with that region's connections to ports in the south and east Asia, which also connected Egypt. Syriac ecclesiastical records also record the outbreak in Antioch and other areas of the eastern Mediterranean, although by then, it may have already spread in the Eastern Mediterranean. During the height of the plague, Constantinople may have lost 5,000-10,000 people per day, although numbers are difficult to determine, and the accuracy of reporting is in question.
Archaeological evidence from Germany and other northern European countries indicate that the disease also spread to these regions. For a while, Northern Europe's relative isolation spared it, but this does not seem to be the case. While many towns and villages had already declined due to the collapse of the Roman Empire, other diseases, and famine, this pandemic further devastated communities.
The Plague hampered Justinian's expansion of the Byzantine Empire
Emperor Justinian, at the time, was busy financing the Hagia Sophia and carrying out his wars in the western Mediterranean. At that point, he was on the verge of retaking critical parts of the Western Mediterranean that would have resulted in the Byzantine Empire effectively reuniting lands from the Roman Empire.
However, Justinian was forced to minimize his campaigns, and likely his forces were weakened. Many crops failed as people were unable to attend to them, and the cost of grain rose sharply. Wages also increased as labor supply became limited, which may have helped to diminish some differences in social standing in the Byzantine and Sasanian Empires.
The economy throughout the Mediterranean faced enormous strain. However, the plague depleted the finances of the Byzantine Empire because it relied on taxation to finance its armies and construction projects. Justinian also became infected with the plague but survived. The Sasanian Empire, great rivals to the Byzantines who were based in Mesopotamia and Iran, were also affected, although we have fewer surviving records.
Initially, they may have benefited, as they were able to make advances on the Byzantines in Armenia and the Levant. However, this began a series of long-term wars with the Byzantines that likely depleted their forces over time. This pandemic enabled the Sasanians to be more easily conquered in the 7th century by the rising Arabs.
The Plague of Justinian dramatically weakened the Byzantine Empire
The immediate consequence of the plague was that it severely limited Byzantine expansion across southern Europe, ending Justinian's dream of reuniting the Roman Empire.
The Goths and Lombards took advantage of this to reconquer and move into territories they had been pushed out of. It also strained the empire's resources as it had to face renewed threats from the Sasanian Empire, all the while Justinian insisted on financing major building projects and imposing taxes on his devastated population.
While the Byzantine Empire ultimately recovered, they did come close to completely collapsing to the Sasanian Empire by the late 6th century and early 7th century. The Byzantine Empire never recovered to the same level again, which suggests the events of the plague may have contributed to the long-term decline despite the short-term recovery of the empire. In northern Europe, the disease likely spread to the British Isles and Scandinavia.
Initially, these regions were isolated and relatively protected from the plague's spread, but slowly it had spread across the area probably by the 540s CE. Northern Europe would not recover until about the 9th century in terms of its population. Diminished trade and contacts would persist between the wealthier parts of southern Europe and northern Europe perhaps for a century or more, which may have also limited the re-urbanization of parts of Europe after the Roman collapse.
However, it may have also resulted in Northern Europe following a different political and social trajectory. ensuring that its history was less affected by the Mediterranean region in subsequent centuries and the Medieval Period, northern Europe developed very differently from the Mediterranean regions. 
Byzantine's finances were hobbled by the pandemic
The main long-term devastation was the economic consequences of the plague. Diminished labor, as well as lost tax revenues, meant that the Byzantines could no longer finance major offensives or construction projects as they had before the plague. The Sasanians likely saw this as an opportunity to expand into Byzantine territory, which did initially succeed. They were able almost to replicate the extent and scale of the Persian Achaemenid Empire, which was seen as the model empire by the Sasanians.
Many of these lands, particularly Jerusalem, Egypt, and Antioch, were critical to the Byzantines. Thus, the Byzantines launched a series of wars to recover these areas. This effort depleted both empires, leading to their weakened states. With these two great empires that effectively controlled the most important cities west of India weakened, the situation did enable the rising Arabs in the mid-7th century to take advantage of the declining powers across the Middle East and eastern Mediterranean. At this point, both those great empires were a shadow of their earlier versions.
For the next two hundred years, historical documents suggest the bubonic plague reoccurred. The re-occurrence of the plague suggests it never went away. The spread of the epidemic to port cities, likely on rats, meant trade was not only diminished, but the plague may have stayed with European populations throughout the Medieval Period. Nevertheless, the rate of death did go down, and most outbreaks were limited to small areas within Europe. The Middle East recovered relatively quickly, and many cities were thriving again in the 7th century.
The last major occurrence of this plague was around 750, which again affected some port areas of the Mediterranean. However, the population may have become more fit to deal with this plague by then, resulting in likely fewer deaths. The eastern Mediterranean would not be significantly affected by pestilence until the Black Death in the 14th century. 
The Plague of Justinian caused about a quarter of the population of the eastern Mediterranean to die. Even those who didn't die were impacted by the plague. The Byzantine Empire, when the plague occurred, was a rising empire spreading its power through southern Europe and the western Mediterranean. It is possible the empire could have reunited the Roman Empire, but the plague ultimately made this dream impossible. While we cannot be sure what would have happened if the epidemic did not occur, the emergence of northern Europe out of its Dark Age may have occurred sooner.
However, another possibility is that northern Europe, as it became isolated, developed its political institutions and relationships, leading to the rise of monarchies in Northern Europe that affected the regions there different from southern Europe and the Mediterranean. This may have led northern Europe to develop a very independent political history that may not have been possible without the plague. It is also possible that conquering Arab armies of the 7th century would have found it harder to defeat both the Sasanian and Byzantine Empires in the 7th century.
- For more on the background to the plague, see: Rosen, W., 2007. Justinian's flea: plague, empire, and the birth of Europe. Viking, New York.
- For more on the events during the plague, see: Little, L.K. (Ed.), 2008. Plague and the end of antiquity: the pandemic of 541-750, Paperback ed. ed. Cambridge Univ. Press, New York, NY.
- For more on the historical impact, see: Bray, R.S., 2004. Armies of pestilence the effects of pandemics on history. Clarke, Cambridge.
- For more on the political and social change that occurred in the 6th and 7th centuries, perhaps resulting from the plague, see: Shepard, J. (Ed.), 2008. The Cambridge history of the Byzantine Empire c. 500-1492. Cambridge University Press, Cambridge, UK ; New York.
- For more on the plague's occurrences, see: Hays, J.N., 2005. Epidemics and pandemics: their impacts on human history. ABC-CLIO, Santa Barbara, Calif, pg. 23.