Fleming was born on a farm at Lochfield in Ayrshire, Scotland and was schooled for two years at the Academy in Kilmarnock. He later attended St Mary's Hospital medical school in London until World War I broke out. He participated in a battlefield hospital with many of his colleagues in the fronts of France. Being exposed to the horrific medical infections by the dying soldiers, he returned to St. Mary's after the war with renewed energy in searching for an improved antiseptic.
Both of Fleming's discoveries happened entirely by accident during the 1920s. The first, lysozyme, was discovered after mucus from his nose dropped into a bacterium laced Petri dish (he sneezed). A few days later, it was noted that bacteria where the mucus had fallen had been destroyed.
Fleming's labs were usually in disarray, which led to be to his advantage. In September 1928, he was sorting through the many idle experiments strewn about his lab. He inspected each specimen before discarding it and noticed an interesting fungal colony had grown as a contaminant on one of the agar plates streaked with the bacterium Staphylococcus aureus. Fleming inspected the Petri dish further and found that the bacterial colonies around the fungus were transparent because their cells were lysing. Lysis is the breakdown of cells, and in this case, potentially harmful bacteria. The importance was immediately recognized, however the discovery was still underestimated. Fleming issued a publication about penicillin in the British Journal of Experimental Pathology in 1929.
Fleming worked with the mould for some time, but refining and growing it was a difficult process better suited to chemists. In part by believing its effect may only hold valid with small infections and further by not being well received within the community, the drug was not developed for mass distribution until World War II when Howard Florey and Ernst Boris Chain developed a method of purifying penicillin to a form that was useful for medical treatment of infection.
For his achievements, Fleming was knighted in 1944 and shared the Nobel prize for Physiology or Medicine in 1945 with Florey and Ernst Boris Chain. Florey was later given the higher honour of a peerage for his monumental work in making penicillin available to the public and saving millions of lives in World War II. Florey's work proceeded over the misgivings of Fleming, who believed that penicillin, for all its intrinsic worth, would not be able to be produced in sufficient quantities to have an appreciable effect in a war situation.
Fleming was long a member of the Chelsea Arts Club, a private club for artists of all genres, founded in 1891 at the suggestion of the painter James McNeil Whistler. Fleming was admitted to the club after he made "germ paintings," in which he drew with a culture loop using spores of highly pigmented bacteria. The bacteria were invisible while he painted, but when cultured made bright colours.
Fleming died in 1955 of a heart attack. He was buried as a national hero in the crypt of St. Paul's Cathedral in London. His discovery of penicillin had changed the world of modern medicines by introducing the age of useful antibiotics.