Polio is not a disease caused by malnutrition. Also, your ancestors probably did not die of it -- it mostly affected young people, so people who died from it usually didn't have descendants.

Quibbling aside, health in general was certainly much worse in our great-grandparents' days, but for those who weren't straight up underfed, nutrition was probably not worse. I'm not sure it was as magically better as some would have you believe, but there is decent evidence that traditional diets were more-or-less fine, and that the modern diet has something wrong with it.