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Unless one attributes the earlier ones to a similar introduction of diseases by ocean-crossers -- which could help clarify where some of the ancient pandemics in Eurasia came from -- one is stuck with the 100 percent natural climate changes that continue to this day.

I think it may be both.

There is convincing proof of temporary settlements of Vikings in Canada who could have brought European diseases to the Americas. Who know where else they may have landed and interacted with American natives for even a day or two.

Transpacific travelers are also likely to have brought disease to the Americas.

Even homegrown pandemics are also inevitable that could drastically reduce population just as they did with the medieval plagues in Europe and Asia.

Changes in weather patterns certainly do cause crop failures and famines. The Dust Bowl crop failures of the 1930s would certainly have caused a famine without the presence of the modern transportation.