In 536 A.D, the Earth experienced what is believed to have been multiple comet impacts that enshrouded the planet in dust for a year.
Sonoran Conservative
Conservative of the Southwest and the musings of the day. – Murum Aries Attigit
In 536 A.D, the Earth experienced what is believed to have been multiple comet impacts that enshrouded the planet in dust for a year.
Sonoran Conservative 
The cooling caused by that dust enabled rats infected with yersinia pestis — Black Death — to survive crossing the desert to reach cities in Egypt. From there the infection was spread by grain ships around the Mediterranean. An estimated 25 million people died from the plague.
Byzantine emperor Justinian caught the plague, but managed to survive. While he was sick, the Empire was ruled by his wife. Unfortunately, she chose to eliminate the “threat” to the throne from Justinian’s favorite un-castrated general, Belisarius, and had him removed from office, stripped of his bodyguards, and exiled from Constantinople. Justinian reversed the punishment a short time after he recovered. However, the damage to the campaign to retake Italy was done, as less competent generals were barely able to hold onto gains Belisarius had made. Loss of manpower due to the plague made further gains nearly impossible.
IMHO, the plague damage was what made the Byzantines and Sassanids (Persians) such easy opponents for Mohammed’s armies just a few generations later. The number of men available for soldiering dropped 90% from pre-plague numbers, and while it would have recovered some, it would not have reached “normal” in time to meet the Arab armies.
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