Edward Jenner, an English country doctor from Gloucestershire, administers what will become known as the world’s first vaccination as a preventive treatment for smallpox, a disease that had killed millions of people over the centuries.

While still a medical student, Jenner learned about rural English farm workers’ and physicians’ observations that people who had contracted a disease called cowpox, which caused blistering on cow’s udders, often did not catch smallpox. Unlike smallpox, which caused severe skin eruptions and dangerous fevers in humans, cowpox led to few ill symptoms in these women.

On May 14, 1796, Jenner took fluid from a milkmaid’s cowpox blister and scratched it into the skin of James Phipps, an eight-year-old boy. A single blister rose up on the spot, but James soon recovered. On July 1, Jenner inoculated the boy again, this time with smallpox matter, and no disease developed. In a publication three years later, Jenner first used the term “vaccine” for the treatment, from the Latin word for “cow.” Doctors all over Europe soon adopted Jenner’s innovative technique, leading to a drastic decline in new sufferers of the devastating disease.

In the 19th and 20th centuries, scientists following Jenner’s model developed new vaccines to fight numerous deadly diseases, including polio, whooping cough, measles, tetanus, yellow fever, typhus, and hepatitis B and many others. More sophisticated smallpox vaccines were also developed and by 1970 international vaccination programs, such as those undertaken by the World Health Organization, had eliminated smallpox worldwide.