English physician Edward Jenner (1749–1823) is credited with inventing the vaccine; however, evidence suggests that vaccination (inoculation of a substance into the body for the purpose of producing active immunity against a disease) was used in China, India, and Persia (present-day Iran) in ancient times.

In modern times, Jenner pioneered the science of immunology by developing a vaccination against smallpox. The English physician was practicing medicine in rural Gloucestershire in 1796 when he observed that dairymaids who had been sick with cowpox did not contract smallpox, suggesting that they had developed an immunity to the often fatal disease, which then occurred in epidemics. Jenner must have been quite certain of his theory: He chose to test it on an eight-year-old schoolboy, James Phipps, whom Jenner vaccinated with matter from cowpox vesicles from the hands of a milkmaid, Sarah Nelmes. Jenner then allowed the boy’s system to develop the immunity he had previously observed in the dairymaids. Several weeks later, Jenner inoculated Phipps with smallpox, and the boy did not become even the least bit ill. The experiment was a success. Jenner continued his experiments for two years and then published his findings, officially announcing his discovery of vaccination in 1798.

As Jenner suspected, vaccines provide immunity by causing the body to manufacture substances called antibodies, which fight a disease. Over the course of the twentieth century, vaccination programs have greatly reduced disease, particularly in developed nations where childhood immunization programs are very effective. By 1977 vaccination had virtually wiped out smallpox.