One comparable situation (and one with even greater estimated devastation) would be the Native Americans as Europeans first started arriving in the Americas. Even before Europeans ever deliberately used them as weapons, the previously unknown diseases swept through Native American populations and some estimates claim that up to 90% of them were wiped out in some areas. To compare, the Black Death killed about 1/3 of the Eurasian population.

Without these half-dozen or so diseases effectively emptying out the continents, Europeans may have had a significantly more difficult time (maybe even to the point of impossibility) gaining a foothold in the New World, at least by force.

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