Alexander Fleming was a Scottish bacteriologist born on August 6, 1881. He is best known for discovering penicillin, the world's first antibiotic, in 1928. This groundbreaking discovery occurred when he noticed that a mold called Penicillium notatum had contaminated one of his petri dishes and was killing nearby bacteria. Fleming's work laid the foundation for modern antibiotics, revolutionizing medicine and saving countless lives. He shared the Nobel Prize in Physiology or Medicine in 1945 with Howard Florey and Ernst Boris Chain for their contributions to the development of penicillin as a treatment for bacterial infections.