And, per Alfred Crosby, not just European diseases but European weeds (a good 60+% of the plant life in the temperate New World is Eurasian in origin) and European animals, both livestock and pests. The combination of disease + weeds + livestock allowed Europeans to virtually replace the Native population in the temperate zone (what Crosby calls the "zone of demographic replacement"), but not in the tropics or in areas where the Native population density was high enough to rebound from disease--namely the Andes and the Valley of Mexico.
100% agree that calling a culture fragile because it can't cope with 90% population loss on top of invasion, mass migration, recurring epidemics, perpetual warfare (both among tribes and between Native Americans and Euroamericans), loss of hunting grounds, and decline in animal populations is rather harsh. I think we should rather stand in awe at the niches the Iroquois, Shawnees, Potawatomis, Cree, Five Civilized Tribes, and others were successfully able to carve for themselves in such an environment.