Of course there is - population-wide genetic variation, as my wikipedia link demonstrates.
Nope, those are not real. There is no "population-wide genetic variation".
You must find it really confusing how smallpox killed off the Indians.
What's confusing about an isolated population that doesn't have variola virus antibodies being more susceptible than a population with antibodies and a much longer generational history of exposure?
I find it really confusing how you think that has to do with genetics...