Alexander Fleming

Alexander Fleming trained as a physician and served in the Royal Army Medical Corps during the First World War, where he witnessed soldiers dying not from their wounds directly but from bacterial infections that the era's medicine had no real way to treat, a devastating pattern that shaped the direction of his later research. He returned to civilian research at St Mary's Hospital in London, working on bacteriology in a laboratory colleagues later described, affectionately, as notoriously cluttered.

In September 1928, returning from a family holiday, Fleming noticed that one of his bacterial culture plates, left uncovered and contaminated while he was away, had grown a mold that appeared to be killing the surrounding bacteria. Most researchers would have simply discarded the ruined, contaminated plate as a failed experiment; Fleming instead examined it closely, recognizing that whatever the mold was producing seemed to be actively destroying dangerous bacteria around it.

Fleming identified the substance as penicillin, but lacked the chemistry resources to purify and mass-produce it himself, and his initial 1929 paper drew little attention from the wider scientific community. It took over a decade, and the urgent pressure of the Second World War, before Howard Florey and Ernst Chain's team developed methods to produce it at scale, making penicillin, the world's first true antibiotic, widely available and saving an estimated hundreds of millions of lives since.

Fleming's discovery depended on noticing and taking seriously something that looked, at first glance, like a ruined experiment rather than a result. For young people, his life is a lesson in paying close attention to genuine anomalies rather than discarding them automatically, and in recognizing that a single observation, however accidental its origin, only becomes useful to the world once other people's persistence carries it the rest of the way.