Fleming saw many soldiers die from infections（感染） in their wounds as he worked in a hospital during World War Ⅰ. This made Fleming decide to find a way to help the body fight infections.
In September 1928, Fleming left some glass dishes on a bench in his laboratory for two weeks. When he came back, he noticed something puzzling. Bacteria（细菌） were growing on all the glass dishes except one. On this dish mould（霉菌） had started to grow — the kind found on old bread. The mould seemed to be giving off something that stopped the bacteria from growing. Fleming called it “mould juice”. He tried it on other bacteria, and it killed them, too. Fleming became wild with joy and named it penicillin.
Unfortunately, Fleming's boss thought he was wasting his time and it was impossible to kill bacteria at that time. Fleming did a few more experiments with penicillin, and he also wrote about it so other scientists could learn about it. However, because no one seemed interested in his discovery, he forgot about penicillin and started to work on other things.
In 1939, Ernest Chain, a scientist, and his boss, Howard Florey, were looking for medicines that could kill bacteria. They discovered Fleming's notes and decided to test penicillin. In 1940, they gave penicillin to some sick mice, who survived later. But those who didn't get it died. Florey declared: “It looks like a miracle!” By 1943, the final tests on humans were finished successfully and the world had its first antibiotic（抗生素） medicine.