Imagine that you’re at the mall and you see someone you don’t like – maybe an ex or that annoying kid from high school – but the mall is crowded that day and you can easily blend into the crowd and escape. Now imagine that the mall isn’t crowded. This unsavory person sees you and walks over. Because there aren’t a lot of people around for you to hide among, now you have to talk to someone you’d really rather avoid. In the case of infectious diseases, what actually happens is your body becomes the feeding ground of an organism whose numbers have been so long controlled by preventative medicine that they have all but faded from cultural memory.