Why did European diseases destroy Native Americans, but not vice versa?

by mirkwoodgdp

So we have this concept of the Colombian exchange, where different things were traded when the two worlds met, including diseases. But it seems like you always hear about the europeans bringing diseases that destroyed the Natives, wiping out entire villages and peoples. Why were there no diseases like that (say smallpox for example) in the opposite direction?

Searocksandtrees