https://www.amazon.com/1491-Revelations-Americas-Before-Colu...
It's been a while since I read it but IIRC, the theory is that some of the pigs the Spanish brought with them got loose and went wild, but brought the flu with them. As wild boar spread so did the disease.
https://www.amazon.com/1491-Revelations-Americas-Before-Colu...
https://www.amazon.com/1493-Uncovering-World-Columbus-Create...
Interestingly, current scientific evidence suggests that this was not at all the "natural state" of North America, and that it was actually a temporary ecological imbalance lasting for a couple centuries after smallpox and other diseases wiped out ~95% of Native Americans after contact from the Europeans -- and suddenly the bison went essentially "unchecked".
Obviously this doesn't change the evil of their almost-extinction or the need for bison as a proper balance -- just that the gigantic hordes of bison that Europeans first witnessed likely isn't the right baseline either.
See "1491: New Revelations of the Americas Before Columbus" for a highly readable account of evidence on both sides from 2006.
[1] https://www.amazon.com/1491-Revelations-Americas-Before-Colu...
For those looking to read it, make sure to get the 400-some page version of this book. There is a similarly named book aimed at grade school kids.
https://www.amazon.com/1491-Revelations-Americas-Before-Colu...
https://www.amazon.com/Before-Columbus-Americas-Charles-Mann...
Mann attempts to reconstruct what the Americas were like before European contact. More importantly, he makes a case that some American Indians had a higher standard of living than Europeans.
More importantly, everything really was mostly "good, peaceful and stable ... until the colonizers" showed up. The disrupting factor were the pandemics that happened; not that one culture was superior than the other.