So, there are a couple of things to parse out here:

Europeans brought diseases that the Native Americans didn't have immunity to and most died from that

First, European diseases exacerbated the genocide(s) that settler colonists perpetuated in the Americas. Native populations that were already weakened by slave raids, warfare, destruction of villages and crops, over-hunting of their traditional lands and so forth suffered out of proportion to disease.

Second, there were indigenous diseases that had an impact on the colonizers -- more here from our FAQ.

I commend to your attention the appropriate section of the FAQ.