That's not true. There were extensive trade routes. Not only that, some tribes, like the Inuit participated in spouse swapping as a means of trading and spreading skillsets.

Counting coup was also practiced regularly and logically would have been a potential vector of disease transmission.

You're right about their lack of animals being an important factor in the lack of deadly diseases comparatively.

Also true that they did not live in crowded conditions comparatively, which would have meant less spread.

They had a better diet than most Europeans during the rise of the big diseases and during colonization (until they started getting starved out, that is). Better because it wasn't based on cereal grain crops and better because it was much more varied.

They had better cleanliness and hygiene overall. Better behavioral practices around sickness too, overall.

They had a less stressful lifestyle than the feudal system provided Europeans.

All these things and more were factors in why the populations of the Americas did not develop the brutal plagues that arose amongst the populations of Europe, Asia, and even Africa.