On 28 September 1928, Scottish bacteriologist Alexander Fleming returned from a summer holiday to find a mould — *Penicillium notatum* — contaminating one of his petri dishes and killing the *Staphylococcus* bacteria around it. Rather than discarding the plate, he recognised what he was seeing.
Fleming published his findings in 1929, but penicillin's clinical potential was only realised a decade later when Howard Florey and Ernst Chain at Oxford isolated, concentrated, and tested it. A police officer named Albert Alexander became the first patient treated in 1941 — he improved dramatically before supplies ran out and he died. Mass production, accelerated by wartime need, was achieved by 1944, saving tens of thousands of Allied soldiers' lives.
Fleming, Florey, and Chain shared the 1945 Nobel Prize in Physiology or Medicine. Penicillin transformed medicine, turning previously fatal infections into minor treatable conditions, and launched the antibiotic era. Antibiotic resistance, a direct consequence of widespread use, is now one of medicine's most urgent challenges.