But the genetic transmission of resistance to an infection would be Lamarckian, unless it was rooted in some genetically superior immune system. Then it wouldn't be specific to any particular disease.
P.S. I think you are referring to measles, not smallpox, with the American Indians. the American Indians never were exposed to measles until it was too late. Most Europeans had constant exposure to it and had developed an immune response. Bam! Not a natural resistance like partial sickle cell anemia does for Africans against malaria.
Actualy, smallpox, although measles is another example. The europeans had been exposed to cowpox and smallpox for generations, the American Indians had not. The Mandan were virtually wiped out by an epidemic of smallpox, as were other tribes.