Sometimes, the greatest discoveries happen when you least expect them—like when Sir Alexander Fleming stumbled upon penicillin. It was 1928, and Fleming, a Scottish bacteriologist, had a notoriously untidy lab. After returning from vacation, he noticed something unusual in one of his Petri dishes filled with Staphylococcus bacteria.

A mold, later identified as Penicillium notatum, had accidentally contaminated the dish. But instead of just ruining his experiment, this mold did something miraculous: it killed the bacteria surrounding it. Fleming realized he was onto something big. He saw that the mold produced a substance that was deadly to harmful bacteria but harmless to human cells. Penicillin, the world’s first antibiotic, was born out of this moment of scientific serendipity.

Fleming’s discovery wasn’t just lucky—it changed the course of medicine forever. In the following years, penicillin would go on to save millions of lives, becoming a key weapon against infections that had once been deadly.