The epidemic of the Black Death is perhaps the most famous case of a disease completely devastating an entire continent. Death estimates range from one third to a half of the entire population of Europe succumbed to the disease completely altering life as the survivors knew it. Yet now some scientists believe that Europeans have inherited a resistance to Aids because of the effects of the bubonic plague.
HIV is the virus that causes Aids and scientists have discovered that some people are simply born with an immunity to it. The reasoning for this is at this point unknown, yet many people are inclined to believe it has something to do with the Black Death. They believe that the genetic makeup of descendants of survivors caused a mutation in future genes that for some reason guards them from the virus.
They reached this conclusion because the mutation is absent in both Africa and East Asian populations. Interestingly, the bacteria that causes Black Death and HIV attack the exact same tissues in the body, leading some to conclude a natural immunity was developed!