There was a hemorrhagic fever in ~1545 an ~1576 that killed tens of millions of people. This is well-documented. The exact nature of this hemorrhagic fever is a major open question in the history of North America, and the natives attested its existence before the Europeans arrived AFAIK.

We know about hantavirus in the southwestern US and Mexico but that seems unlikely to be the source based on its epidemiology. This is one of the most interesting scientific questions about North America, the possibility of a latent hemorrhagic virus that has heretofore not been isolated due to a few hundred years of dormancy.

Smallpox definitely added to the problem, especially in more northern parts of the Americas, but there is substantial evidence of brutal culling by a disease we can’t explain in the southern parts of North America.

1545 is well after European contact and close enough that it seems unlikely to be a coincidence. 1519–1521: Hernán Cortés conquers the Aztec Empire. 1532–1533: Francisco Pizarro conquers the Inca Empire.

Further low 10’s of millions of deaths on its own really doesn’t explain the 90% population drop across several hundred years here. Smallpox killed between 65% to 95% of Native American populations but it was far from alone. We’re talking devastating plague after plague for generations which canceled out the tendency for populations to rebound when competition is low. Something like 200+ million deaths on the conservative side over a few hundred years not just one or two devastating but short lived outbreaks.

> There was a hemorrhagic fever in ~1545 an ~1576 that killed tens of millions of people

I haven’t heard of this - do you have any material to recommend on the subject?

> disease we can’t explain

Disease we can't explain that spread a few decades after European ships full of plagues arrived.

I mean, yeah, sure.

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