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I don't have books in front of me, but it is a fact of nedical history that plagues would regular pass through Europe and hit 20% of the population, even as it did elsewhere. It also equally affected adversaries of great empires. As I recall, one of out own continental congresses was moved or called off due to the fever. As a student of history I have visited graveyards to see people planted in wave of disease.
Having said that, nations don't fall because of plagues. There are other primary reasons.