Today is a significant date in the history of public health. It was on May 14th, 1796, that a new advance in fighting smallpox was established: variolation, the century-old practice of deliberately treating healthy people with a mild strain of the smallpox virus in order to give them first a mild case, and then immunity to that infectious disease, was superseded by vaccination. (来源：专业英语学习网站 http://www.EnglishCN.com)

At the time, much of the scientific community considered vaccination to be little more than a variation on variolation. Theoretically, those inoculated with the smallpox virus would be isolated while they were infectious. In practice, they often ended up spreading the disease to the uninoculated, making variolation (from the Medieval Latin word for "pox") less than successful.

It was Doctor Edward Jenner's genius to scientifically satisfy his suspicion (and observation) that humans who contracted the much less dangerous cowpox gained immunity to the deadly smallpox. After decades of study, on May 14th, Jenner injected a small bit of fluid taken from a cowpox pustule on a milkmaid into the arm of an eight-year-old boy. As Jenner had predicted, the effects suffered by the boy were minimal. And until the doctor's death, 27 years later, Jenner periodically tested the efficacy of his vaccine (from the Latin vaccinus, meaning "of or from cows") on his first patient, who never did develop smallpox.