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Edward Jenner was born in the spring of 1749 in Berkeley, Gloucestershire, the son of a reverend, at a time when smallpox killed about 10 percent of the world’s population and up to 20 percent in towns. As a boy he underwent variolation, deliberate infection with smallpox preceded by purging, bleeding and starvation, and the severe case left him with lasting health problems including hearing loss. At 21 he apprenticed in London under the surgeon John Hunter, who told him, don’t think, try. Back in the countryside as a family doctor, he studied the cuckoo and showed that the newly hatched chick, not the adult, pushes the host’s eggs and chicks out of the nest, using a scoop-shaped hollow on its back that disappears after about 12 days. He then turned to rural lore that milkmaids who caught cowpox were spared smallpox, an idea the Dorset farmer Benjamin Jesty had already acted on with his own family.

In the spring of 1796 Jenner took material from cowpox blisters on the hand of milkmaid Sarah Nelmes and inoculated eight-year-old James Phipps, then later exposed him to smallpox, which failed to take. He repeated the test on 23 more subjects, including his 11-month-old son, and showed the vaccine could pass arm to arm. When the Royal Society declined to publish his paper, he published it himself. Britain later banned variolation and offered the vaccine free. Vaccination work cost him his practice, and Parliament granted him 10,000 pounds and later 20,000 more. He died of a stroke in January 1823 at 73, and in 1980 the World Health Organization declared smallpox eradicated.

• The words vaccine and vaccination come from Latin for the pustules of the cow.
• The hide of Blossom, the cow that infected Sarah Nelmes, hangs in a London medical school library.
• Spain’s Balmis expedition carried live vaccine across the Atlantic by infecting 22 orphan boys two at a time.
• Napoleon had his troops vaccinated and released English prisoners at Jenner’s request, calling him one of the greatest benefactors of mankind.
• Jenner’s final paper to the scientific community was on the migration of birds.