Penicillin growing in a petrie dish, 1943.
© NMeM / Daily Herald Archive / Science & Society Picture Library
Penicillin is an antibiotic, a chemical effective at very low concentrations which can kill or stop the growth of a disease-causing microbe. Penicillin is produced naturally in moulds of the genus Penicillium, from where the drug gets its name. Photograph by James Jarche for Illustrated magazine in 1943.