What the other comments said about Asia and Africa interacting with Europe long throughout history, so there was a lot of direct and indirect exposure on all sides already.
Adding: they did intentionally try to commit genocide on the native populations of America. Europeans used germ warfare through infected items to wipe out tribes and actively displaced the native populations to expand into the land. Disease spread because they made disease spread.
[Like the time they purposely gave small pox blankets](https://www.nlm.nih.gov/nativevoices/timeline/229.html)
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