Humans are born observers. And, while we did not develop the technology to see a virus until an electron microscope was first built less than 100 years ago, for at least a thousand years people have noticed that when people contract -- and survive -- a disease, they build up a resistance to it. And for as long as humans have understood that, they have sought ways to gain that protection with less risk. The early vaccine pioneers, like Edward Jenner and Louis Pasteur, knew that they wanted to give people weaker versions of diseases in order to protect them, but they operated through trial and error, experimentation, and observation. They didn't know what they were fighting against -- only that they could't see it and it was deadly. They moved past variolation (giving someone actual live contagion, such as pus from someone sickened with smallpox), to vaccination, which involves giving attenuated (deliberately killed or weakened) versions of a disease or disease byproduct.
Looking for a kid-friendly version of the story of vaccine development? Try the first installment of our Idongetit Science Tales -- the viruses and vaccines series.