excuse me, but it wasn’t 90% European. Hemorrhagic fever was big in pre-colonial indian populations, as were more than a dozen jungle fever and other assorted zoologic pathogens. Caused, similar to China, and, for instance, Amazonians now, by clearing fields for ag. Former Wandering/gathering groups of usually less than 60 joined larger groups to farm, increasing chances of human contact with animal bacteria and viruseseses, and each other.
“Diseases that were probably present in pre-Columbian America included American leishmaniasis, American trypanosomiasis (Chaga’s disease), roundworms, pinworms, tapeworm, treponematosis, tuberculosis, arthritis, cancer, endocrine disorders, dysentery, pneumonia, rickettsial and viral fevers, goiter, and pinta.”
Although New World indigenous disease was mostly of the chronic and episodic kind, Old World diseases were largely acute and epidemic. Different populations were affected at different times and suffered varying rates of mortality.19 Diseases such as treponemiasis and tuberculosis were already present in the New World, along with diseases such as tularemia, giardia, rabies, amebic dysentery, hepatitis, herpes, pertussis, and poliomyelitis, although the prevalence of almost all of these was probably low in any given group
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1071659/
Understood, however, the preexisting diseases weren’t population reducers in general, yet the European-introduced ones greatly decimated the natives.
It wasn’t even on purpose - the Euros didn’t even realize what had happened to the natives until much later. After that there was some on purpose, but the tales of Europeans using large-scale biological warfare on the natives are mostly SJ fabrications.