The dynamic and powerful Byzantine Emperor Justinian is remembered for having tried to restore the fallen glory of ancient Rome by waging a series of military campaigns to retake lands that had been overrun by barbarian tribes. But, while Justinian's armies were fleetingly successful, another scourge bearing his name was far deadlier. Around A.D. 540, a disease borne by rats in Egypt long the breadbasket of the Mediterranean world spread to the Byzantine capital at Constantinople (modern-day Istanbul) where, by some accounts, it claimed 5,000 lives a day and killed nearly half the ancient metropolis' population. From there, the plague moved east and west, becoming antiquity's most lethal known pandemic. Half a century after it began, between 25 million and 100 million in Europe and Asia had died. Some historians say the damage was so great to the Persian and Byzantine empires that it made them vulnerable to the Muslim conquests of the next century. The devastation the plague wrought may have also ushered in the period now known as the Dark Ages in Europe.