Smallpox was a threat to the health of the population of Britain. There was a world-wide epidemic in 1722, 1723 and 1740 - 42. It killed anyone that had it the rich and poor alike.
People were unaware of the cause of smallpox but they noticed people who caught a mild form of smallpox then recovered and didn't catch it again.
The pus from a smallpox scar was rubbed into a cut on the patient being inoculated by the doctor. But unfortunately, it didn't always work and some of the patients died from the disease as it didn't affect everyone the same way.
Jenner was from Gloucestershire and he had trained as an apprentice to a surgeon-apothecary and then practised medicine in London before returning to Gloucestershire and he was particularly interested in inoculations and he gathered the evidence of over 1,000 cases where smallpox inoculation had failed.
He regularly treated dairy maids for cowpox and he noticed that when there was a smallpox epidemic those who had previously suffered cowpox did not catch smallpox and he decided that they must be connected somehow.
Jenner needed to test his theory and so, in 1796, he infected a local boy, James Phipps, with cowpox and then six weeks later he attempted to infect James with smallpox but he didn't catch it so Jenner infected some more local people with cowpox to test his theory further and it worked so he wrote a book with all his finding called 'An Enquiry into the Cause and Effects of the Variolae Vaccinae'