Alexander Fleming shared the 1945 Nobel Prize in Medicine for the “discovery of penicillin and its curative effect in various infectious diseases.” According to the story as commonly told, a stray spore of Penicillium mold, borne by fortune through an open window, landed on an open bacterial culture in Fleming’s lab in 1928. Fleming later happened to notice a clear zone on the plate, where no bacteria grew (Figure 21.1). “Seeing that halo was Fleming’s ‘Eureka’ moment, an instant of great personal insight and deductive reasoning,” Time magazine celebrated on its website 100 Persons of the Century. “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.” Millions of lives were saved, it seems, based on one astute observation. Fleming’s story reflects an ideal about science: a moment of genius cascades into monumentality. At the same time, the discovery of penicillin is also one of the most famous cases of chance, or accident, in science. Fleming himself often underscored the role of chance in his work. Despite his numerous honors and awards, he was fond of reminding others: “I did not invent penicillin. Nature did that. I only discovered it by accident.” Yet one may recall Pasteur’s famous dictum that “chance favors the prepared mind.” That is, Fleming was surely being modest. He seems not just lucky. He seems exceptionally perceptive. What but intuitive genius could have guided the “great personal insight and deductive reasoning” hailed by Time and marked by the Nobel Prize? The role of “Eureka” moments in propelling scientific progress is certainly ingrained in popular lore—so deeply as to escape questioning. However, with a bit more historical awareness, one may well wish to challenge this sacred bovine. Ironically, the episode exhibits even more “chance” than is often told.