Alexander Fleming discovered penicillin in 1928 after observing mold inhibit bacterial growth.
Fleming returned to his laboratory after a holiday and noticed that a mold contaminating a Staphylococcus culture had created a clear zone where bacteria did not grow. He identified the mold as belonging to the genus Penicillium and named the antibacterial substance penicillin.
Fleming published his findings in 1929, but turning the observation into a practical medicine required later work. Howard Florey, Ernst Chain, and their colleagues developed methods to purify and produce penicillin in useful quantities during the early 1940s. Large-scale production then helped treat wounded soldiers and civilians during World War II.
Penicillin treats susceptible bacterial infections, not viral illnesses such as colds or influenza. Its success began the antibiotic era, although resistance can develop when antibiotics are misused.