Plague of Justinian
The Plague of Justinian was a pandemic of the bubonic plague caused by the bacterium Yersinia pestis. It happened during the years of 541 to 542 in the Byzantine Empire while under the rule of Justinian I. Historians estimate that this pandemic killed as many as 100 million people across the world, partly because it returned every twelve years until 770, when it stopped for about 500 years.