RI contageous OR overpopulation of existing bacteria?

On the one hand, I've heard that RI is very contagious (like a cold). On the other hand, I've read that it is when a certain bacteria that is already in the snake becomes over populated because they are stressed or weakened. Doesn't it seem like these two statements are contradictory doesn't it? Being contagious to me suggests that if the snake encounters a small amount of the sickness externally, it will catch it -- but if it already has a small amount, then that shouldn't matter.

I believe the disconnect you're having is related to the term "respiratory infection". The term itself is a general term and not an actual specific diagnosis. Yes, respiratory infections are primarily caused by bacteria. However viruses, fungal infections, and possibly even parasites could be the cause. The infection's ability to be contagious, as well as the treatment recommendation for the illness, depends on the source of the infection in the first place.