So I saw this article and though it was really relevant to what we are learning this semester! The article explains that by wiping out people with inferior immune systems the Black Plague actually strengthened the immune systems of the population as a whole. It also may account for the current day differences between the immune systems of populations historically exposed to the Black Plague and those that were not.

http://news.sciencemag.org/biology/2014/02/black-death-left-mark-human-genome?rss=1