Answer: The Plague of Justinian
Long before the Black Plague swept through Europe and decimated the population in the 14th century, it was wiping out hordes of people in the Eastern Roman Empire in the 6th century. What we’ve given different names to over the years—the Bubonic Plague, the Black Plague, the Black Death, and other names that telegraph the seriousness of the disease—are all manifestations of the same bacteria: Yersinia pestis.
When humans began gathering in large numbers in early city-states, vermin came along with them to prey on their food stores and refuse. Along with those vermin came fleas (the real carriers of the bacteria); those pests combined with the poor sanitation practices of the day led to frequent and devastating outbreaks of the bacteria. Long before the staggering number of deaths around the 1340s and 1350s in Europe, there was a smaller, but still deadly, outbreak around the 6th century known as the Plague of Justinian caused by the very bacteria that would travel quietly and with great consequence around Asia and Europe for centuries.
Despite Yersinia pestis infections having high mortality rates ranging from 80 to nearly 100 percent in untreated cases depending on the variety contracted (bubonic, pneumonic, or septicemic), Justinian the I (the Roman Emperor at the time of the original outbreak) survived a brush with the plague that would later be named after him.