The production of immunity to a disease by inoculation of an attenuated form of the virus causing the disease.

Named after the English physician Edward Jenner (1749-1823). Jenner exploited the folk knowledge that people who caught cowpox from their cows could not catch smallpox to develop a method of vaccination against smallpox using an attenuated form of the virus. Also known as Jenner’s method and the jennerian method.

The story, briefly told, is as follows. In May 1796 a dairymaid, Sarah Nelmes, consulted Dr. Jenner about a rash on her hand. He diagnosed cowpox and Sarah confirmed that one of her cows, a Glouchester cow named Blossom, had recently had cowpox, a mild infection of cows that causes a some weeping spots called pocks on their udders, but nothing more. Milkmaids occasionally caught cowpox from the cows. Although they developed a small number of pocks, usually on the hand, and they might become mildly ill, the disease did not otherwise trouble them. It was also known that cowpox could pass not only from cow to person from person to person.

Jenner decided to test the protective properties of cowpox by giving it to someone who had not yet had smallpox. He selected James Phipps, the 8-year-old son of his gardener and on May 14 made a few scratches on James’ arm 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. To test whether the cowpox would protect James from smallpox, on July 1 Jenner variolated him by scratching into his skin scab material from someone with smallpox. James did not develop smallpox then or later.

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