This year’s flu season has been reported as one of the worst in nearly a decade. And that means the flu shot is everywhere, from doctor’s offices to drugstores. We take for granted that the flu vaccine is commonplace now…and that we don’t have to worry about diseases like smallpox anymore. But just two centuries ago, millions of people were dying of all kinds of brutal diseases – and it took massive scientific exploration to develop the vaccinations that eradicated them