Theodor Billroth (1829–1894) is best known as the inventor of subtotal gastrectomies (removal of parts of the stomach), known as Billroth I and II, which are still performed today. Billroth’s investigations in the field of bacteriology are less well-known. These investigations were prompted by a clinical incident in 1867 when he was treating a patient suffering from breast cancer. He discovered bacteria that he called streptococci. During further investigations, he clearly recognized the possibility of an inhibitory effect of a mold (Penicillium) on streptococci but cautiously interpreted it as the effect of a “change in the medium”. The inhibitory substance turned out to be penicillin in the 1940s and opened the field of antibacterial therapy (chemotherapy). The discoverers Fleming, Florey, and Chain made medical history and were awarded the Nobel Prize for their work. A honeydew melon, a clever laboratory assistant, and a geneticist played a major role in ensuring that 180 tons of penicillin were produced and made available to doctors by the end of the Second World War.