Whose discovery of penicillin changed medicine?

In 1928, Alexander Fleming returned to his London laboratory to find that a mold, Penicillium notatum, had contaminated a dish of bacteria and killed the microbes around it. He identified the substance it produced as penicillin. Fleming struggled to purify it, and it was Howard Florey and Ernst Chain who, around 1940, turned penicillin into a usable drug and mass-produced it in time to save countless lives in World War II. The three shared the 1945 Nobel Prize. Penicillin launched the antibiotic era, transforming once-deadly infections into treatable conditions.