Edward Jenner's 1796 experiment, which involved inoculating a boy with cowpox to provide immunity against a deadlier disease, created the world's first what?

Vaccine

Jenner's breakthrough came from observing that milkmaids who caught cowpox seemed immune to the much deadlier smallpox virus. The word 'vaccine' actually comes from the Latin word 'vacca', meaning cow, in honor of the animal used in this pioneering experiment. This discovery eventually led to the global eradication of smallpox in 1980.