In 1796, doctor Edward Jenner gave an eight-year-old boy material from a cowpox sore. Cowpox was a milder infection than the disease he hoped to prevent: smallpox.
After the boy recovered, Jenner deliberately exposed him to smallpox. He did not develop the disease.
This dangerous test became one case in Jenner’s evidence for vaccination. He tested protection by trying to cause the disease it was meant to prevent.