ORIGINAL: crsutton
And just suppose that the flow of deadly pathogens did not go from the old world to the new but the reverse happened-wiping out 80% of the population of Europe before the first Meso-American ever set foot ashore in France. Would we still be practicing human sacrifice today?
Not gonna happen.
Simple explanation? Almost all 'old world' plagues are, in fact, animal diseases. Diseases from *domesticable* animals (yes, there are exceptions ... Malaria, for example, which is probably originally a disease of Monkeys dating well after the split between Humans and Apes, but they are mostly much less lethal).
The diseases jump the species barrier because they are domesticated animals ... and it was common for humans and such animals to live in close proximity (hence China being an influenza sink because Birds, Pigs and Humans live in close proximity in rural areas) which made it effectively inevitable.
The problem is, in the New World, there are very few domesticated/domesticable species ... the Llama/Alpaca, the Guinea Pig, the Chihuaha etc. ... hence there is less chance that such animal diseases as existed in those populations would spread, especially as the Llama/Alpaca is geographical isolated within the New World.
Then there's the problem of how these diseases jump the species barrier and then become endemic with occasional epidemics ... it requires lots of humans living close together. Cities, in effect. And lots of time ... thousands of years.
Again, the Old World has an advantage ... almost all of the domesticable food crops, and all of the most efficient ones (in terms of grain or edible part size/yield) are from Eurasia. The few that are elsewhere available are generally developed much much later than in the Old World and, hence, cities are a much later development in the New World ... so, even if they'd had more domesticable species, they'd not have had enough time to develop the whole range of nasties that the Old World had in its arsenal.
Phil