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By the time Justinian's plague had run its course in AD 590, it had killed as many as 100 million people -- half the population of Europe...

This seems unlikely if the entire world population was only 200 million.

[blam]

From the article linked in post #17:

"Then came pestilence. The Justinian plague, named for a Byzantine emperor, apparently began in central Asia, spread into Egypt, and then swept across Europe. Hundreds of thousands died.