The first antibiotic discovered in the 20th century was penicillin. Discovered by Alexander Fleming in 1928, penicillin is derived from the mold Penicillium notatum. Its discovery marked a revolutionary advancement in medicine, providing an effective treatment for bacterial infections and saving countless lives. Penicillin paved the way for the development of various other antibiotics, transforming modern healthcare and significantly reducing mortality rates associated with infectious diseases.