Penicillin

Penicillin is the first true antibiotic — a compound made by Penicillium mold that kills bacteria by wrecking their cell walls while leaving human cells untouched. Alexander Fleming noticed its effect in 1928 when mold contaminating a discarded culture dish killed the bacteria growing around it. A decade later it was purified into a life-saving drug.

Before antibiotics, a scratch, a childbirth, a strep throat, or a battlefield wound could kill through infection, and doctors could do little but wait. Penicillin made death from common bacterial infection largely optional. It arrived in time to save vast numbers of Allied lives in the Second World War.

Fleming's original observation was nearly thrown out as a spoiled experiment. Turning mold juice into a usable drug fell to Howard Florey and Ernst Chain at Oxford, who purified it around 1940. Wartime mass production was then cracked in the United States, partly by fermenting a far more productive mold strain found on a moldy cantaloupe in Peoria, Illinois.

Penicillin launched the antibiotic age, making surgery, chemotherapy, transplants, and childbirth dramatically safer. It set the template for hunting new drugs inside microbes and reshaped medicine within a single generation. Its overuse also created the modern problem of antibiotic resistance.