Disease immunity in America

I'm curious what it would take for there to be enough pre-Columbian contact for the Native Americans to have immunity to Afro-Eurasian diseases equivalent to what Europeans had developed (ie- not as much as Africans had to Smallpox and malaria). I'm not saying a situation like Vikings in 1000 CE which would probably collapse population centers and wouldn't give enough time prior to 1492.

Is there a way to have Polynesian, African, and Bering Sea contact be just enough to spread the diseases early enough? But without causing the New World to be "known" in European circles and cause Columbus not to sail. I'd still like to see Columbus on target for 1492.
 
Polynesian-nope. Polynesia did not have problems with rampant diseases. In fact, they were about as vulnerable as Native Americans to epidemics of disease. Same with contact across the Bering sea, the Chukchi and other Siberian peoples were quite isolated from China and Japan where Eurasian diseases were likely to run rampant.

African contact would definitely spread malaria, possibly smallpox and possibly yellow fever. However, having West Africans with both that kind of seafaring capability reach the Americas before the 16th century while ignoring or not effecting Europe seems quite difficult.
 
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