The precursor to modern methods was to obtain a little bit of smallpox and inoculate yourself with it. It was vastly less deadly than being exposed in the norma

The precursor to modern methods was to obtain a little bit of smallpox and inoculate yourself with it. It was vastly less deadly than being exposed in the normal course, though not really safe. It still demonstrated the principle, that a little bit of smallpox was quite a lot safer than being exposed to a lot of it. This is not a good idea generally, it worked on smallpox when there was no other option. Still, dose can absolutely make a difference in how deadly a virus is. https://en.m.wikipedia.org/wiki/Variolation https://en.m.wikipedia.org/wiki/Variolation The first real vaccine was to inoculate with cowpox instead, which was safe for humans but induced immunity to smallpox as well. Modern methods are substantially safer, usually by entirely deactivating the virus before using it, but there are still some vaccines that use live virus (the oral polio vaccine is a live attenuated virus).