Most of this happened centuries before the germ theory of disease.

No one on either side had any real clue what was going on, and most probably thought it was a cruel punishment from the God(s).

Often the disease travelled faster than people, and when Europeans arrived, the natives were long since gone.

there are recorded incidents: https://project1492.org/small-pox-blankets/

the controversy is mostly around whether or not it was a systemic, pervasive form of biological warfare. the evidence base is fairly small but needs to be considered in context of Native American cultural habits around durable recordkeeping (eg trade ledgers) vs colonial, and if colonists would record such an act

there was also active ethnic cleanses and genocides committed by the US government repeatedly throughout US history too and those are very well documented, all dotted with stories as bad or actively and intentionally worse than biological warfare

https://news.uoregon.edu/content/historian-examines-native-a...

"He was speaking in opposition to a treaty that proposed the Cherokees sell off 20 million acres of homeland—a large portion of present-day Kentucky and Tennessee. This tension exploded with the commencement of independence hostilities in July 1776; some Cherokee leaders sided with the British, and in response the US charged thousands of colonial troops with “the utter extirpation of the Cherokee Nation.”

https://sites.uab.edu/humanrights/2017/04/17/indian-removal-...

"Throughout American history, the treatment of indigenous Native Americans has violated numerous articles of the United Nations Universal Declaration of Human Rights. These violations resulted in the loss of numerous Native American homelands, the Cherokee being only one example, and the genocide of numerous other smaller tribes since the beginning of European colonization. This is largely due to Eurocentric ideals, like the natural law of the Puritan worldview, which elevates the status of European peoples over that of indigenous, Native American peoples through a biased worldview. This mindset is so pervasive and powerful that it still prevails today, evidenced by modern films and television that paint Native American tribes as savage, ignorant and of ill intent toward the “white man”, and the policies of the current United States government."

see also: https://en.wikipedia.org/wiki/Native_American_genocide_in_th... or https://en.wikipedia.org/wiki/List_of_Indian_massacres_in_No... or https://en.wikipedia.org/wiki/Denial_of_genocides_of_Indigen...

> A lack of resistance and no sanitation

Precolombian north america actually had really sophisticated sanitation. In urban areas they pretty advanced waste management. In other areas, a lot of their seasonal mobility was possibly driven by sanitation needs. And they have really in depth practices around the ways they treat waterways and waste placement.

The problem was actually the exact opposite. Compared to Europeans, they were obsessive about soap and cleanliness. But Europeans lived in really dense villages alongside their livestock for many generations and they suffered greatly from it. The history of Europe is basically one outbreak after another. But ofc over time some amount of immunities developed

Maybe if North Americans had less sanitation and kept livestock they would've developed their own deadly diseases too that would've killed Europeans.