Alexander Fleming, a Scottish bacteriologist in London, discovered penicillin by mistake when he was trying to study staphylococci bacteria in 1928. He was running experiments with the bacteria in his laboratory at London's St. Mary's Hospital, and set a laboratory dish containing the bacteria near an open window. Upon returning to the experiment, he found that some mold blown in through the open window onto the dish, contaminating the bacteria.

Instead of throwing away his spoiled experiment, Fleming looked closely at it under his microscope. Surprisingly, he saw not only the mold growing on the staphylococci bacteria, but a clear zone around the mold. The Penicillium mold, the precursor to penicillin, was dissolving the deadly staphylococci bacteria.

Fleming was originally optimistic that penicillin would be useful as an antibacterial agent, as it was safe for the human body, yet potent. Later, in 1931, he changed his mind and decided that penicillin would not last in humans for the duration needed to kill harmful bacteria, and stopped studying it. In 1934, he began another few years of clinical trials and tried to find someone else to purify it.

Researchers at Oxford University in England, including Howard Florey, Ernst Chain, and Norman Heatley experimented with Fleming's discovery of penicillin. They proved that penicillin would be both harmless and effective in mice, but did not yet have the volume of penicillin needed to treat people. Orvan Hess and John Bumstead were the first people to use penicillin to successfully treat a patient.

Penicillin saved the lives of many soldiers in World War II, but the supply was extremely limited, and the drug was rapidly excreted from the body, so the patients had to be dosed frequently. It was common practice at this point to save the urine from patients undergoing penicillin treatment so that the penicillin could be isolated and reused. Another agent, probenecid, was eventually found to prolong the duration of penicillin in the human body.

Fleming, Florey, and Chain were awarded the Nobel Prize in medicine in 1945 for the discovery and development of penicillin. Florey was openly worried about the possibility of a population explosion resulting from health care improvements and said that his work with penicillin was more of an interesting scientific problem than a way to help people. He did admit that the fact that it could help people was a good thing, but not why he was originally interested.

Andrew J. Moyer later discovered how to make large quantities of penicillin, patenting the process and advancing the fight against infectious diseases. In 1987, he was inducted into the National Inventors Hall of Fame for his process of creating penicillin in high quantity.