By and large improved health care won't affect population resistance all that much, it just improves survival rate. In an earlier time you would have been right, European germs caused more Native American deaths than Europeans themselves when they first colonized, but nowadays with global trade we all end up sharing the same diseases for the most part.

Sedentary lifestyles enabled by modern lifestyles as opposed to a hunter-gatherer lifestyle can contribute to decreased overall health but thats not the issue here since almost nobody is a hunter-gatherer anymore.

When it comes to new viruses, all of us are in the same boat immunity-wise.