Variolation (introducing the smallpox virus through the skin, not respiratory tract as its natural spread would be, usually leading to a milder infection and subsequent immunity) had been around for a while. Jenner’s accomplishment was successfully using the related cowpox virus to grant immunity to the smallpox virus, based on observations that people working with cattle rarely caught smallpox. This eliminated many of the downsides of variolation (eg. risk of breakthrough disease, and variolated individuals being infectious for smallpox for a while).