This is one of the most frequently asked questions on this sub; there is a good answer by /u/anthropology_nerd in this older thread. The short of it is that European diseases didn't spread so widely among Native American populations just because their immune systems weren't familiar with the germs; it was because their immune systems had been systematically weakened by war, famine, enslavement, and the total disruption and dislocation of their way of life as an immediate consequence of European encroachment on their lands. Since the same factors did not pertain in Europe, there was no comparable mass death on the European side.