British bacteriologist Alexander Fleming (1881–1955) made one of the most important medical discoveries of the 20th century—penicillin—on this day in 1928… and it happened by accident!

When a staphylococcus culture became contaminated with mold, he noticed the mold stopped bacterial growth. He later explained, “One sometimes finds what one is not looking for.”

The active substance—penicillin—killed harmful bacteria. With help from other researchers, this natural antibiotic soon began saving lives during World War II, treating diseases like pneumonia, syphilis, diphtheria, and spinal meningitis.

Fleming's sharp mind met serendipity. He was knighted in 1944 and awarded the Nobel Prize in 1945. Yet he remained humble: “Nature makes penicillin. I just found it.”

Hungarian biochemist Albert Szent-Györgyi said it best: “A discovery is said to be an accident meeting a prepared mind.”