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Why didn’t the Europeans acquire new diseases when they explored the Americas?(r/askscience)
This.
OP, if you’re interested in these kinds of questions, you should read Guns, Germs and Steel by Jared Diamond. The book looks a bit intimidating, but it’s really easy to read and he goes right to the root of the inequalities in wealth, technological development, social organization, etc. between the old and new worlds. (Spoiler Alert: It ain’t about genetic superiority.)