Penicillin was discovered in 1928 by Alexander Fleming, a Scottish bacteriologist. Fleming noticed that a mold called Penicillium notatum produced a substance that killed nearby bacteria in his laboratory at St. Mary's Hospital in London. He published his findings, but it wasn't until the late 1930s and early 1940s that penicillin began to be mass-produced and widely used as an antibiotic. This discovery revolutionized medicine, significantly reducing deaths from bacterial infections, and marked the beginning of the antibiotic era. Fleming's work paved the way for further research and development of other antibiotics, fundamentally changing the treatment of infectious diseases.