In the late 19th century, a greater understanding of disease led researchers to seek treatments to kill or inhibit the growth of microbes. This proved elusive until the accidental discovery of penicillin in 1928 by Alexander Fleming. A culture plate of Staphylococcus bacteria from an experiment was left on his bench during a two-week vacation and became contaminated with Penicillium notatum mold. When Fleming returned and found the plate, he noticed that surrounding the yellow-green mold growth was a distinct clear halo. Since the Staphylococcus bacteria was able to grow across the plate except near the section contaminated with mold, Fleming postulated that Penicillium was somehow capable of inhibiting bacterial growth.

It took another 11 years for an Oxford University research team, under the guidance of biochemists Ernst Chain and Howard Florey, to isolate and purify penicillin. In a 1940 Lancet paper they reported that mice injected with a lethal dose of Streptococcus could be cured with penicillin. Initial testing on patients began immediately, and in a 1943 Lancet article Florey described incredible success using penicillin to treat wounded soldiers in North Africa. European production of penicillin was limited by fighting during World War II, but by D-Day, June 6, 1944, US pharmaceutical companies were able to produce enough penicillin to treat all wounded British and American troops. Penicillin quickly became a primary treatment for pneumonia, diphtheria, syphilis, gonorrhea, scarlet fever, and many other infections. For the discovery of penicillin and its curative effect in various infectious diseases Fleming, Chain, and Florey received the Noble Prize for Medicine in 1945.