Alexander Fleming discovered penicillin in 1928. Fleming, a Scottish bacteriologist, accidentally stumbled upon the antibiotic properties of the mold Penicillium notatum while working on bacterial cultures. He observed that the mold inhibited the growth of surrounding bacteria, which led to his research into its potential applications in medicine. Penicillin became the first true antibiotic used to treat various bacterial infections and is credited with saving countless lives, especially during World War II. Fleming's discovery revolutionized the field of medicine and opened the door to the development of many more antibiotics.