Discovered in 1928, Penicillin was one of the world’s first antibiotics, but the man who discovered it—Dr. Alexander Fleming—never actually meant to “revolutionize all medicine,” as he later described it. Rather, Fleming came across the antibiotic entirely by chance when he left out cultures of Staphylococcus aureus in his lab for two weeks and returned to find that their growth had been prevented by a mold called Penicillium notatum.