What discovery did Edward Jenner make in 1796?

Edward Jenner's discovery in 1796 was pivotal in the history of medicine, specifically concerning vaccinations. He observed that milkmaids who had contracted cowpox—a less severe disease—seemed to be immune to smallpox, a deadly and contagious illness. To test this observation, Jenner inoculated a young boy with material taken from a cowpox sore. The boy developed a mild case of cowpox and, crucially, did not contract smallpox when exposed to it later.

This landmark work led to the development of the smallpox vaccine, the first successful vaccine ever created. Jenner’s methodology and findings laid the groundwork for modern immunology and vaccination practices.

The other options refer to developments that occurred later in medical history. The germ theory of disease, which established that microorganisms cause many diseases, was formulated in the late 19th century, long after Jenner's time. Anesthesia was introduced for surgical purposes in the 1840s, and the principles of antisepsis were developed in the 1860s by figures like Joseph Lister. Each of these advancements contributed significantly to medical progress, but they were distinct from Jenner's original work with smallpox and the beginnings of vaccination.