Who discovered penicillin?

Alexander Fleming, in 1928, by leaving a culture plate out while he went on holiday and noticing on his return that a contaminating mould had cleared a zone around itself. He published, could not purify or stabilise the substance, and largely moved on - and for about a decade penicillin was a laboratory curiosity.

It became a drug because of Howard Florey, Ernst Chain and Norman Heatley at Oxford, who worked out how to extract and concentrate it and tested it in 1940. They were so short of material that they recovered it from patients' urine to re-use. Their first patient, a policeman with an infection from a scratch, improved dramatically and then died when the supply ran out. Wartime Britain could not build the plant, so Florey and Heatley took it to America, where deep-tank fermentation turned it into an industrial product in time for D-Day. The three shared the Nobel Prize; Heatley, whose bench methods made it possible, did not.

Fleming himself warned about resistance in his Nobel lecture in 1945, before most doctors had used the drug - that it would be easy to underdose, that bacteria would be educated to resist it, and that a man might do so at home. It is one of the most precisely accurate warnings in the history of medicine, and it was published while the thing being warned about had barely begun.