The improbable chain of events that led Alexander Fleming to discover penicillin in 1928 is the stuff of which scientific myths are made. Fleming, a young Scottish research scientist with a profitable side practice treating the syphilis infections of prominent London artists, was pursuing his pet theory–that his own nasal mucus had antibacterial effects–when he left a culture plate smeared with Staphylococcus bacteria on his lab bench while he went on a two-week holiday. When he returned, he noticed a clear halo surrounding the yellow-green growth of a mold that had accidentally contaminated the plate. Unknown to him, a spore of a rare variant called Penicillium notatum had drifted in from a mycology lab one floor below. Luck would have it that Fleming had decided not to store his culture in a warm incubator, and that London was then hit by a cold spell, giving the mold a chance to grow. Later, as the temperature rose, the Staphylococcus bacteria grew like a lawn, covering the entire plate–except for the area surrounding the moldy contaminant. Seeing that halo was Fleming’s “Eureka” moment, an instant of great personal insight and deductive reasoning. He correctly deduced that the mold must have released a substance that inhibited the growth of the bacteria. It was a discovery that would change the course of history. The active ingredient in that mold, which Fleming named penicillin, turned out to be an infection-fighting agent of enormous potency. When it was finally recognized for what it was–the most efficacious life-saving drug in the world–penicillin would alter forever the treatment of bacterial infections. By the middle of the century, Fleming’s discovery had spawned a huge pharmaceutical industry, churning out synthetic penicillins that would conquer some of mankind’s most ancient scourges, including syphilis, gangrene and tuberculosis. Fleming was born to a Scottish sheep-farming family in 1881. He excelled in school and entered St. Mary’s Hospital in London to study medicine. He was a short man, usually clad in a bow tie, who even in his celebrity never mastered the conventions of polite society. Fleming probably would have remained a quiet bacteriologist had serendipity not come calling that fateful September in 1928. In fact, Fleming was not even the first to describe the antibacterial properties of Penicillium. John Tyndall had done so in 1875 and, likewise, D.A. Gratia in 1925. However, unlike his predecessors, Fleming recognized the importance of his findings. He would later say, “My only merit is that I did not neglect the observation and that I pursued the subject as a bacteriologist.” Although he went on to perform additional experiments, he never conducted the one that would have been key: injecting penicillin into infected mice. Fleming’s initial work was reported in 1929 in the British Journal of Experimental Pathology, but it would remain in relative obscurity for a decade.