Edward Jenner started researching vaccinations using the cowpox vaccine

The only defense against smallpox was a crude vaccine technique called variolation, which included purposefully infecting a healthy person with "matter" removed from a patient suffering from a minor case of the illness. Unfortunately, the transmitted illness did not always stay mild, and occasionally there was a fatality. The individual who received the vaccination may also spread the illness to others, serving as a point of infection.

The fact that a person who had experienced an attack of cowpox, a condition that may be caught from cattle, could not take smallpox, i.e., could not become infected whether by accidental or deliberate exposure to smallpox had impressed Jenner. Jenner considered this phenomenon and came to the conclusion that cowpox was an intentional defense system that not only prevented smallpox but also provided protection from it.

The tale of the important discovery is well known. In May 1796, Jenner discovered Sarah Nelmes, a young dairymaid, who had recently had cowpox blisters on her hand. James Phipps, age 8, who had never had smallpox, was vaccinated on May 14 using material from Sarah's lesions. Over the following nine days, Phipps experienced a mild illness, but he was OK by day 10. On July 1, Jenner gave the boy another immunization, this time using smallpox material. No illness spread; full immunity was provided. An Inquiry into the Causes and Effects of the Variolae Vaccine was a slim book published privately in 1798 by Jenner after he added more instances.