Sir Alexander Fleming was a young bacteriologist when an accidental discovery led to one of the great developments of modern medicine on September 3, 1928. Having left a plate of staphylococcus bacteria uncovered, Fleming noticed that a mold that had fallen on the culture had killed many of the bacteria. He identified the mold as penicillium notatum, similar to the kind found on bread. In 1929, Fleming introduced his mold by-product called penicillin to cure bacterial infections.