What accident most changed the course of history?

Laurence Gonzales, journalist and author

In 1928, a dish of Staphylococcus bacteria in the lab of Alexander Fleming was accidentally contaminated with Penicillium notatum mold. Ernst Boris Chain and Howard Walter Florey took up the development of penicillin in the 1930s, and the three shared a Nobel Prize for their work in 1945. The number of lives that have been saved by the drug is impossible to know.