When the Columbian Exchange happened, why was it only Europeans spreading the disease and natives catching it, and not the other way around?

by Discopotato3

For example, smallpox spread from Europeans to the native people and decimated their population, but why did the reverse not happen, where the Europeans arrived and caught some deadly disease with no built up immunity?

jelvinjs7

More can be said if anyone wishes to add anything, but in the meantime, there is a FAQ section on misconceptions and reality surrounding Native American vs European diseases. In particular, you may appreciate:

When Europeans brought diseases to the new world, how come Europeans themselves didn't get sick from diseases specific to the new world? by u/anthropology_nerd
Why weren't European settlers in North America decimated by disease like the native populations? by u/400-Rabbits