A smallpox epidemic broke out causing 35,000 to die.
There were a lot of dairy farms where Jenner did work. He treated diary maids for cow pox. Cow pox was a disease which caused red blisters on the skin.
Jenner needed to test the theory so he infected a local boy, James Phipps, with cowpox but he didn't catch it. He also infected more local people.
His work inspired other scientists like Pasteur and Koch to search for more vaccinations.
Pasteur's first effort for a vaccine was chicken cholera. He identified that it worked to vaccinate the chickens.
By 1900, scientists all over the world starting separating microbes and inventing new vaccinations.