The Plague of Justinian was the first bubonic plague pandemic in history that was reliably recorded, and it lasted for more than two centuries. Experts estimate that between 30 and 50 million people died during this plague, which was almost a quarter of the upper limit of the human population.

The Plague of Justinian is named after Justinian I, who was the emperor of the Byzantine Empire when the plague began. In 542 CE, the plague – which had been traveling along maritime trade routes – reached Constantinople, the capital of the Byzantine Empire, according to Procopius, a well known historian of the Byzantine Empire.

Based upon Procopius’ description of the symptoms of the plague and more recent DNA analysis, experts have concluded that the culprit of the Plague of Justinian was the bubonic plague, although pneumonic and septicemic plague could have also been around at the time. 

Like the Black Death–another bubonic plague that would ravage Europe and Asia almost 1,000 years later–the bubonic plague was spread by black rats, especially the fleas carried by them.

Some experts also believe that humans spread the plague among themselves, too, and that the plague wasn’t just being spread by rats and fleas. During the Plague of Justinian, people had poor hygiene habits, and cities were especially unsanitary, so humans carried around their own fleas and lice, which could have been vectors for the bubonic plague. 

Many experts believe the Plague of Justinian was primarily spread by humans because of how rapidly the plague spread across the empire–many experts believe it spread too quickly for rats to have been the only spreaders of the disease.

Experts estimate that the Plague of Justinian originated in China or India and was brought to Egypt and then throughout the rest of the Byzantine Empire by land and sea trade routes. Although the Mediterranean region was most affected by this plague, there were reports of outbreaks of the bubonic plague as far as London and Germany during the sixth century.

It is estimated that 20 to 40 percent of Constantinople’s population was killed by the plague, while about 25 percent of the empire's population died in total due to the plague. It makes sense that Constantinople was disproportionately affected by the plague because of the population density in cities and the number of rats in cities that allowed the plague to spread rapidly. 

Constantinople was also the center of trade in the Byzantine Empire, and the plague went where trade went, which further contributes to why Constantinople was so hard-hit by the plague.

At the height of the pandemic, Procopius reported that 10,000 people were dying daily in Constantinople alone. Historians today disagree with Procopius’ reporting and place the estimate at closer to 5,000 deaths per day. Suffice to say, the plague spread through Constantinople and the Byzantine Empire like wildfire.

This plague significantly impacted the Byzantine Empire both politically and economically. The Byzantine Empire had been weakened by the plague and was unable to effectively fight off enemies. The high death toll from the plague weakened the military simply because many members of the military died, but the military was also weakened because many of the people who assisted the military, for example, the people who supplied food, also died.

Furthermore, the military was unable to recruit and train new soldiers due to the plague. Since the strength of the military had been substantially weakened, the Byzantine Empire was unable to fight off the Lombards, who invaded northern Italy, or the Arabs, who invaded North Africa and the Near East, which is considered to be the region of southwest Asia near the Red Sea, the Black Sea, and the Caspian Sea. Thus, the plague is largely responsible for the Byzantine Empire's decreasing in size. 

Additionally, the Plague of Justinian also hurt the Byzantine Empire’s economy. Constantinople, like many cities in the empire, seemed to grind to a halt. The economy was devastated and the plague wreaked havoc on trade. 

The agriculture industry in particular suffered because of the high death tolls. This meant that there were fewer people to feed; resultantly, the price of grain skyrocketed, and tax revenues decreased since there were significantly fewer people working. 

However, Emperor Justinian increased taxes on those that were still living in an effort to restore the Roman Empire to its former glory. Consequently, the economy was further devastated by the high taxes. As the plague ravaged the empire, Justinian I continued to wage wars to fight off enemies, which put an additional strain on the economy due to the costs associated with wars.

Overall, the Byzantine Empire suffered an incredibly high death toll from the Plague of Justinian as well as from economic and political issues caused by the plague. Over the next 200 years, the plague reoccurred in waves all over the empire, especially in cities. Since the plague lasted until about 750 CE in the Byzantine Empire, it took the empire and the population a long time to recover.

The world would not experience a plague pandemic like the Plague of Justinian until the Black Death almost 1,000 years later; this later plague would once again devastate Europe, which is estimated to have killed around 25 million people in Europe alone.