To test his theory, Dr. Jenner took material from a cowpox sore on milkmaid Sarah Nelmes’ hand and inoculated it into the arm of James Phipps, the 9-year-old son of Jenner’s gardener. Months later, Jenner exposed Phipps several times to variola virus, but Phipps never developed smallpox. More experiments followed, and, in 1801, Jenner published his treatise “On the Origin of the Vaccine Inoculation.” ... Vaccination became widely accepted and gradually replaced the practice of variolation. At some point in the 1800s, the virus used to make the smallpox vaccine changed from cowpox to vaccinia virus.