In the summer of 1928, at St Mary's Hospital in London, the Scottish bacteriologist Alexander Fleming carelessly left staphylococcus cultures behind before going on holiday. When he returned in September, a mould — Penicillium notatum — was growing in one Petri dish, with the bacteria around it dissolved. Many scientists would have tossed the contaminated plate; Fleming examined it.

He named the substance produced by the mould "penicillin" and published a short paper in 1929. But he was unable to isolate and purify the compound, so clinical use had to wait a decade. Between 1939 and 1941, at Oxford, the team of Howard Florey, Ernst Chain, and Norman Heatley worked out the methods; under wartime pressure, mass production began in the United States. By the Normandy landings, penicillin given to the wounded saved tens of thousands of soldiers' lives.

The world after penicillin was a different place. Pneumonia, meningitis, septicaemia, post-surgical infection, and syphilis — once often death sentences — became treatable. In 1900 the leading causes of death in industrialised countries were infectious diseases; by 1950 it was heart disease and cancer. Average human life expectancy lengthened by decades over the second half of the 20th century.

Today antibiotic resistance is one of the most serious public-health issues in the world. Bacteria adapt; new antibiotic discovery has slowed. In his 1945 Nobel lecture Fleming warned of exactly this: "the person who plays with penicillin treatment is morally responsible for the death of the man who succumbs to infection with the penicillin-resistant organism."