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Transmission and Spread

Updated: Mar 30, 2021

During the onset of this particular strain of the Bubonic plague, the Roman empire had nearly reached its greatest extent. Spanning from what would later be the southernmost point of Spain to the far east reach of Jerusalem.

While it would not reach the span detailed in the picture above until 555CE, it nonetheless details this bacteria's ability to travel and propagate among a (relative to today) small population. Scholars and studies on the topic have detailed the first cases reported in 541CE, meaning within two decades, this strain of the plague managed to cover three continents, and both infect and kill anywhere from 30-50 million people. This equals approximately half of the world's known population at that time.


The people of that time may not have realized the benefits of protective masks and handwashing. Still, regardless, it is a testament to the durability and contagious ability of this bacterium. Compared to today, travel to these wide-reaching locations was something of an ordeal. Without the assistance of planes, trains, and automobiles, even a journey between two metropolises like Rome and Constantinople would entail covering a distance of nearly 1400km as the crow flies. A brisk walk of 35km a day in a direct line would see our intrepid journeyman arrive at their destination within a month, but that hardly details the reality of what travel back then would entail, and if we assume these travelers were hosts of the bacteria then what percentage of these people could survive not only the journey but the disease itself to further the spread?


All these variables make it unlikely that the average person was responsible for the widespread infection of the plague of Justinian. Certainly, once it reached a city, it would be a simple task for people to do as they will and spread it to whoever they meet, but as luck would have it, there stands what would become a classic place to lay blame for all things plague related:

Yes, the common rat was long held as the primary culprit for all things disease-related. They would board ships in great numbers, survive the voyage to port cities, breed and spread the disease, and then repeat the cycle. Of course, modern understanding places less burden upon these simple rodents, putting them at no more fault than the filthy peasants of the time. The greater villain has since been revealed to be something even smaller than rodents.

Known as a threat to dogs and elementary school children everywhere, fleas, both figuratively and literally, bite. This, paired with fleas acting as carriers of Yersinia pestis (the bacteria responsible for the plague), make them an extremely effective method of transmitting the plague from host to host. The fleas are carried by the rats, spreading the disease to them, and then the fleas latch onto humans, further spreading to them. From there, people take the reins to spread the fleas themselves, making a terribly vicious cycle.


As mentioned, common foot-traffic from city to city was likely not the cause of most infections of the plague. The largely held belief is that it was spread via sailors, ships, and the infected therein. Data suggests that the bacteria's probable origin within the Roman Empire was Alexandria of Northern Africa, where the Empire's greatest granaries were stored. As its wares were shipped across the Empire, the bacterium stowed away and was inadvertently taken to all corners of the otherwise unassuming world.

And so, with all these factors playing unique roles, it becomes easy to imagine how a perfect environment for a disease to propagate was made. The success of Yersinia pestis spreading throughout the Roman Empire made for disastrous consequences for the Empire at large and the people living within its borders.


Read all about these failures in the post; The Bad and the Ugly


Or, brush up on the real villain of this Plague;

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