"The doctor made a few scratches on one of James' arms and rubbed into them some material from one of the pocks on Sarah's hand. A few days later, James became mildly ill with cowpox, but was well again a week later. So Jenner knew that cowpox could pass from person to person as well as from cow to person.
"The next step was to test whether the cowpox would now protect James from smallpox. On July 1, Jenner variolated the boy. As Jenner anticipated, and undoubtedly to his great relief, James did not develop smallpox on this occasion nor on the many subsequent occasions when his immunity was tested again."