The outbreak may have originated in Ethiopia or Egypt and moved northward until it reached metropolitan Constantinople. The city imported massive amounts of grain to feed its citizens—mostly from Egypt—and grain ships may have been the original source of contagion, with the massive public granaries nurturing the rat and flea population.
The Byzantine historian Procopius records that, at its peak, the plague was killing 10,000 people in Constantinople every day, although the accuracy of this figure is in question and the true number will probably never be known for sure; what is known is that there was no room to bury the dead, and bodies were being left stacked in the open. The Byzantine Emperor Justinian I ensured that new legislation was swiftly enacted so as to deal more efficiently with the glut of inheritance suits being brought as a result of the plague deaths (Moorhead, J., 1994).
Justinian had expended huge amounts of money for wars against the Vandals in the Carthage region and the Ostrogoth Kingdom of Italy. He had also dedicated significant funds to the construction of great churches like the Hagia Sophia. Amidst these great expenditures, the plague's effects on tax revenue were disastrous. As the plague spread to port cities around the Mediterranean, it gave the struggling Goths new opportunities in their conflict with Constantinople. The plague weakened the Byzantine Empire at the critical point at which Justinian's armies had nearly wholly retaken Italy and could have credibly reformed the Western Roman Empire. It also may have contributed to the success of the Arabs a few generations later in the Byzantine-Arab Wars.
The long term effects on European and Christian history were enormous. Justinian's gambit was ultimately unsuccessful. The overextended troops could not hold on. When the plague subsided, they were able to retake Italy but not to move further north. They held it for the remainder of Justinian's life, but the empire quickly lost it after he died. Italy was decimated by war and fragmented for centuries as the Lombard tribes invaded the north.
Ancient historians did not hold to modern standards of fact-checking or numerical accuracy. The actual number of deaths will always be uncertain. Modern scholars believe that the plague killed up to 5,000 people per day in Constantinople at the peak of the pandemic. It ultimately killed perhaps 40% of the city's inhabitants. The initial plague went on to destroy up to a quarter of the human population of the eastern Mediterranean. New, frequent waves of the plague continued to strike throughout the 6th, 7th and 8th centuries AD, often more localized and less virulent. It is estimated that the Plague of Justinian killed as many as 100 million people across the world. Some historians such as Josiah C. Russell (1958) have suggested a total European population loss of 50 to 60% between 541 and 700.