It’s not surprising at all. If you assume naively that development rate of a novel disease is proportional to population, then the World, which had a 6:1 greater population would have 6 times as many communicable diseases. Similar argument if you base it off of land mass, number of wild animals, number of domesticated animals, etc.
(Actually, I do think the New World peoples were particularly prolific when it came to domesticating plants… they punched way above their population size in terms of number of today’s staple foods they domesticated… plus chocolate, vanilla, etc…)
I’ve read that most human viruses jumped from domesticated animals. The pre-Columbian American peoples notoriously had almost no domesticated animals, with I think just one exception being the llama. So I think that’s supposedly the primary factor, less so raw population.
> ”Similar argument if you base it off of… number of domesticated animals…”
But again, I think that fact isn’t surprising, either, considering the Old world is much larger and had more wild animals and more humans than the New World.
You also had millions of years for diseases to evolve to infect people in the Old World. Then there were fairly small populations that traveled to the New World. If they didn't bring the diseases with them, there was only about 10,000 years for disease evolution, and a much smaller population for much of that time.
