In advance, I'm sorry if this is an extremely common question you have here.

So, anyways, during the Columbian exchange, from what I've learned, the conquering Europeans had developed immunity to diseases common in Europe, that, when brought into the Americas, wroke utter havoc on the Native Populations. Is there a certain reason Native Americans didn't have immunity to certain diseases Europeans had never come across? Or was it just pure coincidence?

EDIT: Please think of this as a repost, as shortly after posting this, I came across a question thread on this website that answered the above.