Alexander Fleming is usually credited with discovering penicillin. The familiar story has him returning from holiday in 1928 to find a Petri dish contaminated with mold, with the bacteria around the mold visibly dead. He published the observation in 1929. The story is true. It is also incomplete. Several earlier scientists had made essentially the same finding, and each time the discovery had been quietly buried.

Around 1870, the British physician John Burdon-Sanderson published observations of antibacterial activity in molds, including Penicillium. The work was noted in medical journals, briefly discussed, and forgotten. In the same decade, Joseph Lister — the founder of antiseptic surgery — reportedly observed similar effects in his own laboratory at King's College Hospital and used the mold to treat a patient with a severe wound infection. He moved on to other questions, and his notes on the subject were never developed into a published program.

In 1897, a young French medical student named Ernest Duchesne defended a doctoral thesis at the École du Service de Santé Militaire de Lyon on the antagonism between molds and bacteria. He used Penicillium extracts to cure infected animals. The thesis was filed in the archive and never followed up. Duchesne contracted tuberculosis and died in 1912 at the age of 37, before he could pursue the line of research.

When Fleming made the same discovery in 1928, he too struggled to develop it. He believed the active compound was too chemically unstable to make a useful drug, and turned to other work.

It was Howard Florey and Ernst Chain at Oxford, between 1939 and 1941, who finally figured out how to purify and produce penicillin at clinical scale. By 1944, it was treating Allied wounded in Europe and the Pacific. The basic science had been visible to careful observers for nearly 70 years. The medicine was missing because the development effort, not the original discovery, was the hard part.