05/31/2026
He went on holiday. He came back to a dirty petri dish. And what he saw on that dish eventually saved over two hundred million lives.
But the most remarkable part of the story is not the discovery. It is what happened in the ten years after β when the man who found penicillin could not turn it into medicine, and nearly let it disappear entirely.
Alexander Fleming was born on August 6, 1881, at Lochfield Farm near Darvel in Ayrshire, Scotland β the seventh of eight children born to a hill farmer. The land there was green and steep, the air sharp and cold, and the work never finished. Growing up on that farm taught him something that no classroom could have replicated β the habit of watching. How things grew. How they failed. How the natural world rarely behaved in straight lines.
He moved to London as a teenager and worked in a shipping office for several years. Then an inheritance from an uncle, and the encouragement of an older brother already working in medicine, changed his path. He enrolled at St. Mary's Hospital Medical School in London. He qualified as a doctor in 1906 and spent the rest of his career at St. Mary's β never leaving the institution where he had trained.
The First World War broke something open in him.
He served as a captain in the Royal Army Medical Corps, working in field hospitals in France. The soldiers he treated were not dying from bullets. They were dying afterward β from the infections that settled into their wounds while they lay in hospital beds waiting to recover. Between twelve and fifteen percent of men treated in front-line hospitals died from bacterial infections. Gas gangrene. Blood poisoning. Wound infections that consumed men who had survived everything the battlefield had thrown at them.
The antiseptics being used β carbolic acid, chlorinated solutions β were doing real damage. They killed bacteria on the surface of wounds but destroyed healthy tissue at the same time, often making deep wounds worse rather than better.
Fleming watched men die from infections he believed medicine should have been able to stop. He left the war determined to find something that could.
In 1921, seven years before penicillin, he made his first significant discovery β and it came from an accident.
He had a cold. A drop of his own nasal mucus fell onto a culture plate of bacteria. He noticed, when he checked it later, that where the mucus had landed the bacteria had dissolved. He investigated further and found that mucus, tears, saliva, and even egg white all contained a natural antibacterial substance. He named it lysozyme.
It was not strong enough to kill most dangerous bacteria. But the discovery built something in Fleming that would matter far more than the finding itself β the habit of taking accidents seriously.
Most researchers would have wiped the contaminated plate clean and started again. Fleming looked at the unexpected thing and asked why.
In September 1928, he left his laboratory at St. Mary's for a two-week summer holiday in Scotland. Before leaving, he stacked several petri dishes of Staphylococcus aureus β a bacteria that causes boils, abscesses, and dangerous wound infections β on a bench near a window. They were not placed in an incubator. They were not sealed.
He returned on September 3.
While sorting through the stack of dishes, one of them stopped him.
A mold had grown on the plate. Around the mold, in a clear ring visible to the naked eye, the bacteria were gone. The zone was completely clean β as if something in the mold had reached outward and dissolved everything near it.
He did not wipe it away. He did not move on.
He set it down carefully and looked at it for a long time.
He identified the mold as belonging to the genus Penicillium β the same family found on stale bread and aging fruit. The substance it was producing, he called it informally mold juice. In 1929, he published a paper on the finding, naming the active ingredient penicillin. His paper described the mold's antibacterial properties clearly and accurately.
And then, for the next decade, almost nothing happened.
Fleming tried repeatedly to isolate and purify penicillin into a stable form β something that could be used as a medicine in a human body. The chemistry defeated him. The substance broke down too quickly. His laboratory lacked the equipment and the expertise to take it further. He kept small samples going, offered penicillin occasionally to colleagues for surface treatments, but gradually set the larger project aside.
The discovery sat in a published paper that almost no one read.
For ten years, penicillin waited.
In 1938, a German-born biochemist named Ernst Chain, working at Oxford University under the Australian pathologist Howard Florey, came across Fleming's neglected 1929 paper while researching lysozyme. Florey and Chain, together with colleagues Norman Heatley and Edward Abraham, spent two years extracting, purifying, and testing penicillin β first in the laboratory, then in mice.
The results in mice were extraordinary.
Their first human patient was treated in January 1941. He was Albert Alexander, an Oxford police constable dying from a severe infection that had begun from a scratch caused by a rose thorn. The infection had spread to his face, his scalp, and his lungs. He had been deteriorating for months with no effective treatment available.
Within days of receiving penicillin, he began to recover.
The supply ran out. The team collected his urine, extracted the penicillin passing through his body, and gave it back to him. But there was not enough. He relapsed. He died on March 15, 1941.
The principle, though, was proven beyond doubt. Penicillin worked in a human body. The only problem was producing enough of it.
British pharmaceutical companies were stretched to their limits by wartime production. Florey and Heatley flew to the United States in 1941. The American government declared penicillin a war project. Pharmaceutical companies including Merck, Pfizer, and Squibb poured resources into scaling production using deep-tank fermentation techniques that allowed the drug to be made in quantities no one had previously thought possible.
By D-Day β June 6, 1944 β penicillin was available across Allied battlefields.
The mortality rate from wound infections, which had been twelve to fifteen percent in the First World War, fell to approximately three percent in the Second. Men who would have died in 1918 from infected wounds were walking out of field hospitals in 1944.
In December 1945, Fleming, Florey, and Chain accepted the Nobel Prize in Physiology or Medicine together in Stockholm.
Fleming was generous in his acceptance speech. He acknowledged the essential work that Florey and Chain had done in turning his observation into medicine. He was clear that the discovery alone would have meant nothing without the decade of scientific work that followed it.
But his Nobel lecture also contained something else β a warning that was precise, specific, and has turned out to be one of the most accurate scientific predictions of the twentieth century.
He told the audience that misuse of penicillin β taking too little, using it when not needed β would allow bacteria to develop resistance to it. He described a future in which penicillin could be purchased without a prescription. He warned that an uninformed person might take too small a dose, expose bacteria to a non-lethal quantity of the drug, and inadvertently help those bacteria learn to survive it.
He was describing antibiotic resistance in 1945.
The world is still reckoning with that warning today.
Fleming was knighted in 1944. He died of a heart attack on March 11, 1955, in London. He was seventy-three years old. He is buried in St. Paul's Cathedral.
He had grown up watching how the natural world worked on a hillside farm in Scotland. He had carried that habit of observation into a laboratory for forty years. And on a September morning in 1928, when most scientists would have disposed of a contaminated dish without a second glance, he looked at something unexpected and asked the question that changed medicine permanently.
The discovery almost disappeared. The paper sat unread for a decade. The man who made the finding could not finish what he had started. And yet penicillin survived β because enough people, at the right moment, paid attention to what had been quietly waiting for them.
For those who have ever put careful work into something and watched it sit unnoticed β and then seen it finally reach the people it was meant for β Fleming's story is a reminder that good work does not always disappear. Sometimes it simply waits.
What is something you worked on carefully that took far longer than it should have to be recognized?