Not only is natural selection kicked in, and I feel bad for saying it, but it's true. You have people now that are passing along genes with a very stronger immune system that are going to be able to survive for longer. The life expectancy is going to go up, not to mention that you have a better diet now. You have a surplus of food because you have less people that are eating. Wages are going to go up as well because there's less workers that are available. So you're going to have kind of a burgeoning middle class that eventually is going to spread from this. Not to mention that not in the too far off distance, the Black Death is going to result in a questioning of the Catholic Church if they couldn't stop this.