KIt stems from smallpox back in the 1700s. Inoculation was achieved by injecting a small amount of smallpox to get the body to react and develop immunity. I think back then people were scratched with rather the needles we know today. Jenner discovered that using weaker or attenuated cowpox into someone the body developed immunity for smallpox. The Latin word for cow is vacca.....so says it all really
Today vaccines use attenuated, or modified viruses, just as Jenner discovered, to get an immune response from the body