Alexander Fleming notices a bacteria-killing mold growing in his laboratory, discovering what later became known as penicillin.

Here are four paragraphs on Alexander Fleming’s discovery of penicillin:

Alexander Fleming’s discovery of penicillin in 1928 was one of the most important breakthroughs in the history of medicine. Fleming was a Scottish bacteriologist working at St Mary’s Hospital in London, where he studied bacteria that caused infections. At the time, bacterial infections such as pneumonia, scarlet fever, and infected wounds could be extremely dangerous because there were few effective treatments. Fleming had already become interested in substances capable of killing bacteria without harming human tissue, an interest that eventually led to his famous discovery.

The discovery occurred largely by accident. After returning to his laboratory following a holiday in September 1928, Fleming noticed that one of his Petri dishes containing *Staphylococcus* bacteria had become contaminated with a mold. Instead of simply throwing the dish away, he examined it carefully and noticed something unusual: the bacteria surrounding the mold had been destroyed, leaving a clear area around it. Fleming realized that the mold was producing a substance capable of killing or preventing the growth of certain bacteria. The mold was identified as belonging to the genus *Penicillium*, and Fleming named the antibacterial substance it produced “penicillin.”

Fleming published his findings in 1929, but his discovery initially attracted relatively little attention. He found it difficult to isolate and purify penicillin in sufficient quantities for use as a medicine, and for about a decade its potential remained largely unrealized. In the late 1930s and early 1940s, researchers including Howard Florey, Ernst Boris Chain, Norman Heatley, and their colleagues at the University of Oxford developed methods for extracting, purifying, testing, and producing penicillin. Their work demonstrated that the drug could successfully treat serious bacterial infections in humans. During the Second World War, British and American researchers and pharmaceutical companies developed techniques for producing penicillin on a large scale.

Penicillin transformed modern medicine by providing doctors with a powerful treatment for infections that had previously killed or seriously disabled millions of people. It proved especially valuable for treating infected wounds, pneumonia, septicemia, and other bacterial diseases, and its success encouraged scientists to search for additional antibiotics. Fleming, Florey, and Chain jointly received the 1945 Nobel Prize in Physiology or Medicine for the discovery of penicillin and its therapeutic effects. Fleming also warned that improper or insufficient use of antibiotics could encourage bacteria to develop resistance—a problem that has since become a major global health concern. His chance observation in 1928, combined with the later work of many other scientists, helped usher in the modern antibiotic era.