Penicillin was discovered by accident in 1928, in a mould that grew on a forgotten laboratory dish.
Why it is true
Returning from holiday, Alexander Fleming noticed that bacteria had died around a mould of the genus Penicillium. Only during the Second World War did Howard Florey and Ernst Chain manage to produce it in quantity. All three received the Nobel Prize in 1945.
Source: Nobel Prize Outreach
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