According to About.com, bacteriologist Alexander Fleming discovered penicillin in 1928. After returning from vacation, he noticed that mold had formed in a discarded Petri dish set aside from a contaminated experiment, and the mold had killed the bacteria contained in the Petri dish.
Biography states Fleming was born in Scotland. He studied bacteriology in London and served as a field doctor during World War I. His study of wound infections and treatments led him to human immune system research. For his penicillin discovery that opened the doors to the use of antibiotics in modern health care, Fleming was awarded the Nobel Prize in 1945.