English physician Edward Jenner had long observed that dairy workers who caught cowpox seemed immune to smallpox, a disease that killed a third of its victims and disfigured many survivors. He took material from a lesion on milkmaid Sarah Nelmes and inserted it into two small cuts on the arm of his gardener's son. Weeks later he exposed the boy to smallpox itself; Phipps did not fall ill, proving the principle of vaccination.
Smallpox killed an estimated 400,000 Europeans every year in the late 18th century and left countless survivors blind or scarred. Jenner's insight, borrowed from rural folk wisdom that dairymaids rarely caught smallpox, was to test the protective power of cowpox systematically rather than dismiss it as superstition.
The experiment itself is startling by modern ethical standards: Jenner inoculated a healthy child with pus from an infected woman's blister, waited six weeks, then deliberately exposed him to smallpox to see if he would survive. James Phipps did survive, and Jenner repeated the test on more subjects before publishing his findings in 1798, coining the term 'vaccine' from the Latin 'vacca,' for cow.
The World Health Organization declared smallpox eradicated in 1980, the only human disease ever eliminated, a direct descendant of the crude but brilliant experiment Jenner ran in his garden in Gloucestershire. Phipps was later given a house and land by Jenner and remained free of smallpox for the rest of his life.