Sir Alexander Fleming is known for discovering penicillin in 1928. His accidental discovery occurred when he noticed that a mold, Penicillium notatum, contaminated one of his petri dishes and had killed surrounding bacteria. This finding revolutionized medicine, leading to the development of antibiotics that have saved countless lives. Penicillin became the first true antibiotic used to treat bacterial infections, significantly reducing mortality rates from infections that were once often fatal. Fleming's groundbreaking research laid the foundation for the field of antimicrobial therapy and has had a lasting impact on modern medicine.