One of the reasons why Europeans became so immune to these new diseases was that they lived absolutely filthy lives with their farm animals back in Europe. Native American tribes noted how much these new foreigners smelled when they first met.
And the only reason why they survived those deadly diseases was not because they had stronger immune systems ..... millions of people had to die first and it was the survivors who carried on. The diseases that flourished was a fine balance between being infectious enough to spread but not so deadly so as to kill people too quickly to stop the spread.
First Nation people weren't exactly ultra clean themselves but they kept a distance from wanting to live with a bunch of live animals in their living space.