Justinian's Plague (541-542 CE)

"During the reign of the emperor Justinian I (527-565 CE), one of the worst outbreaks of the plague took place, claiming the lives of millions of people. The plague arrived in Constantinople in 542 CE, almost a year after the disease first made its appearance in the outer provinces of the empire. The outbreak continued to sweep throughout the Mediterranean world for another 225 years, finally disappearing in 750 CE."

"Procopius reported in his Secret History of nearly 10,000 deaths per day afflicting Constantinople. His accuracy has been questioned by modern historians who estimate 5,000 deaths per day in the capital city. Nonetheless, 20-40% of the inhabitants of Constantinople would eventually perish from the disease. Throughout the rest of the empire, nearly 25% of the population died with estimates ranging from 25-50 million people in total."