How did Jenner make the link between cow pox and smallpox?

Edward Jenner made the link between cowpox and smallpox through his observation that milkmaids who contracted cowpox, a disease affecting cattle, seemed to be immune to smallpox. In 1796, he tested this hypothesis by inoculating a boy with cowpox pus and later exposing him to smallpox; the boy did not develop the disease. This experiment demonstrated that cowpox provided protection against smallpox, leading to the development of the first vaccine. Jenner's work laid the foundation for immunology and the eventual eradication of smallpox.