From the perspective of Native Americans, a number of very bad things happened as a result of the Columbian Exchange. The worst, by far, was that Native peoples were exposed to diseases of European origin for which they had no immunity. These included smallpox, typhus, measles, and various forms of plague. The result was what some historians call a "virgin soil epidemic" that wiped out millions of people. European accounts are full of horrific descriptions of the effects of these diseases. Here is English trader John Lawson, who traveled throughout North Carolina in the early eighteenth century, describing the effects of smallpox on the Sewee Indians:
the Small-Pox has destroy’d many thousands of these Natives, who no sooner than they are attack’d with the violent Fevers, and the Burning which attends that Distemper, sling themselves over Head in the Water, in the very Extremity of the Disease...
Estimates of death tolls vary, but all agree that European diseases were devastating and destabilizing to Native populations and culture.
Another negative effect of the Columbian Exchange for Natives was the introduction of pigs and other livestock that destroyed Indian crops. The effect of these animals was compounded by European weeds that flourished in the New World, also causing problems for Native agriculturalists.