For example, when the Spanish made contact with the vast inca empire, why did they not succumb to exotic diseases the native people had immunity to?
There's always room for more discussion (and either myself or /u/anthropology_nerd would be happy to field follow-up questions), but in the meantime you may be interested in our FAQ section on Disease in the Americas.
I've also expanded a bit on this and provided additional links in recent post on this topic.