My understanding (which I am stating in case you find it errant) is that while there were no domesticable animals in the Americas, the East had a history of many plagues received from livestock coupled with their terrible hygiene. Thus while Europeans already had herd immunity from centuries of diseases, when they came to the Americas the natives were hit with them all at once, subsequently being all but wiped out.
On Reanimated History it is stated that the vast majority of the native population (like 95%) died after very early explorers came to the new world (sorry I could only find the first half of the skit where Adam says this) and that the following plagues were mere outbreaks in a post-apocalyptic society. Other sources such as Quora give mixed answers to this; however, I don't trust Adam Ruins Everything or Quora. I trust Stack Exchange.
Furthermore, why didn't the Europeans realize that they were spreading disease? A couple of other web pages talk about Europeans who did know that they were spreading disease, and actually used it as an act of war; however, all of these happened 200 years after 1492, leaving the question of how people before and after responding.
I think it matters to know both when the Native Americans died, and how much the Easterners knew about what was happening, and if they could have helped.