This day (May 14th) in 1796, Edward Jenner administered the first successful vaccination against smallpox, which was also the first vaccination ever. His work likely saved more lives than the work of any other human with the possible exception of Normal Borlaug.
Vaccines are considered to be the second most important public health development of all time. Public sanitation, being the first, allowed people to have cleaner water and sewer systems.
These two developments are responsible for extending the lifespan of humans as well as the quality of their lives.
Before preservation methods they used to do arm to arm transfer of early vaccines. In some cases they'd use orphans to "transport" long distances. It is remarkable when you all consider this was occurring while germ theory of disease was still generally butting heads against miasma theory.
There were contemporaneous anti-vaxxers back then, which were a bit more understandable than their modern counterparts. Arm to arm transfer in some cases would lead to contamination from other pathogens like syphilis. The early smallpox vaccine was unattenuated vaccinia virus and could have severe complications in some individuals. But smallpox was so deadly and aggressive that it was a clear trade-off.