> The reason native Americans didn't spread epidemics to Europeans, rather than the other way around, is because Europeans had domesticated animals. The diseases of animals jumped to humans after so much exposure.
The reason was that European peoples had exposure to some kinds of disease that peoples in the Americas did not. Civilizations in the Americas had domesticated animals for quite some time, just different kinds of animals.
If that were the only factor, then the diseases of the Americans would likewise have wiped out the arriving Europeans.
The Europeans had much more hard-core diseases because of a mix of factors (including animal domestication), but the chief one was population density.
European diseases evolved in tandem with the people themselves -- each time something swept through the population and crippled/scarred/killed large numbers of the people, the folks with some level of natural immunity would be favored. The diseases would evolve (at a rapid pace, due to reproducing at enormous scale...), as would the people (very painfully).
People in the Americas were mostly more widely spread out. A nasty variant on a disease that wipes out a village and stops there goes extinct.
There were some cities, trading between areas, etc., but not on a European scale.
(Source: "Guns, Germs and Steel", which went into this all in fascinating detail... though admittedly I read it years ago now).
You are correct, from my understanding, in that a variety of factors from population density, to agriculture and animal domestication led to a greater spread of diseases. Even in analysis of native american populations before European contact we see that diseases increased when population density increased [1].
The other factor that hasn't been mentioned is that a disease can be deadlier when the population density is higher. The disease can find another host to infect before the original dies and still stay active. Whereas in a situation with spread apart populations the deadlier variant would die out once the small local population was killed.
The European diseases carried this much deadlier profile and thus were more effective in killing native americans than native american diseases were in killing americans.
Then why didn't the invading Europeans die from new American diseases in equal proportions? Because old-world livestock lived closer to humans, in larger quantities, for longer, therefore giving Europeans many more diseases to become immune to.
Basically, Europeans had domesticated animals on a much larger scale than a few llamas a few places on the continent. It was deeply integral to the old-world way of life in a way that Native American culture didn't even come close.
The reason was that European peoples had exposure to some kinds of disease that peoples in the Americas did not. Civilizations in the Americas had domesticated animals for quite some time, just different kinds of animals.