Estimates of the Plague of Justinian's death toll in Constantinople 541
The plague takes its name from Emperor Justinian I, who ruled the Byzantine Empire (Eastern Roman Empire) at the time of the outbreak. The Byzantine capital of Constantinople (also known then as Byzantium, and Istanbul today) was the hardest hit city during the pandemic, and where the majority of sources are from. Until recently, it was only assumed that the outbreak was bubonic plague, due to the symptoms described by contemporary historians, but scientists were able to confirm that it was in fact Yersinia pestis (the bacterium that causes plague) in 2013.
The Plague of Justinian was the first major bubonic plague pandemic recorded in Europe, and was the first pandemic to ever be described or documented with any relative reliability.