September 29, 2025

— Sir Alexander Fleming

(If you see my dad today, wish him a happy birthday!)

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'Nature did that'

Professor Alexander Fleming was not what you'd call a tidy scientist.

His laboratory at St. Mary's Hospital in London was famously cluttered: petri dishes stacked everywhere, cultures growing in every available corner, the organized chaos of a brilliant mind at work.

His colleagues often joked about the state of his workspace, but Fleming's relaxed approach to housekeeping would prove to be one of medical history's most fortunate accidents.

In September 1928, Fleming had just returned from vacation at his country home in Suffolk, and he was cleaning up some old bacterial culture plates that had been sitting on his laboratory bench for several weeks.

His experiments at the time had to do with Staphylococcus bacteria—the kind that causes everything from skin infections to pneumonia.

Now, something caught his eye.

One of the dishes had been contaminated by a blue-green mold during his absence, that had somehow drifted in through an open window. This should have been cause for frustration—his work potentially ruined by airborne spores.

But, Fleming noticed something remarkable: wherever the mold had grown, the surrounding bacterial colonies had been killed off, leaving clear zones like halos around the fungus.

"That's funny," Fleming reportedly said to his assistant.

From Curiosity to Cure

Fleming realized he was looking at something unprecedented—a naturally occurring substance that could kill dangerous bacteria. The mold, he would later identify, was Penicillium notatum, a common fungus related to the kind that grows on old bread.

Now, many scientists might have simply discarded the contaminated plate and started fresh.

Fleming's genius however lay in recognizing the profound implications of what seemed like laboratory carelessness. He immediately began studying the mold's properties, growing pure cultures and testing its effects on different bacteria.

He found that the "mold juice," as he initially and rather unpalatably called it, was remarkably effective against Staphylococcus, Streptococcus, and other dangerous bacteria.

But it was harmless to human white blood cells.

Fleming named the active substance penicillin after the mold that produced it, and published his findings in 1929.

But, the medical community initially showed little interest. The substance was difficult to purify and seemed unstable. For over a decade, penicillin remained a laboratory curiosity.

The War That Changed Everything

But, World War II transformed penicillin from scientific footnote to medical miracle.

In 1940, a team at Oxford University led by Howard Florey and Ernst Boris Chain began working to develop Fleming's discovery into a practical medicine. The urgent need to treat wounded soldiers provided both motivation and funding for their research.

By 1942, penicillin was being mass-produced. The results were immediate and stunning: infections that had been death sentences became treatable conditions. Soldiers who would have died from infected wounds survived to return home.

The mortality rate from bacterial pneumonia, which had killed one in three patients, dropped to less than one in twenty.

By D-Day in 1944, Allied forces had enough penicillin to treat all their wounded—a strategic advantage that undoubtedly saved thousands of lives and helped win the war.

The Numbers Are Staggering

Conservative estimates suggest that penicillin and the antibiotics it inspired have saved over 200 million lives since 1942. Some researchers put the number even higher—as many as 500 million people who would have died from bacterial infections but didn't.

Every major bacterial killer that terrorized previous generations—tuberculosis, pneumonia, sepsis, infected wounds, childbirth fever—suddenly became treatable.

Routine surgeries became safer.

Childhood infections that once claimed whole families became minor inconveniences.

The CDC estimates that before antibiotics, one in nine people who developed a serious skin infection died. One in three people who developed pneumonia died. Women regularly died in childbirth from bacterial infections.

Now these conditions kill a fraction of a percent of patients in developed countries.

A Personal Revolution

But numbers tell only part of the story. Fleming's accidental discovery touched virtually every family in the developed world.

That grandmother who survived pneumonia at 85? She might have Fleming to thank.

The child who recovered quickly from a strep throat infection? The parent who came through surgery without complications? The countless people who never even knew they were at risk because antibiotics prevented infections before they could take hold?

All of them owe something to a Scottish bacteriologist who kept a cluttered office, and who was too curious to throw away a contaminated petri dish.

Fleming himself seemed amazed by what his observation had unleashed.

"I did not invent penicillin," he said years later. "Nature did that. I only discovered it by accident."

The Ongoing Gift

Today, we face new challenges—antibiotic resistance means we must use these drugs more carefully than previous generations. But Fleming's fundamental insight—that nature provides tools for healing if we're observant enough to notice them—continues to guide medical research.

September 28, 1928 is the date Fleming recalled discovering the mold. If we wanted to get pedantic, there's some debate over whether his calendar was as messy as his office, and if he therefore might have discovered his breakthrough a few weeks earlier.

Still, it reminds us that sometimes the most important discoveries happen not when we're looking for them, but when we're wise enough to recognize them when they find us.

And sometimes, the difference between a ruined experiment and a world-changing breakthrough is simply being curious about why something looks "funny."

Two hundred million lives saved, and counting. All because a scientist paused to wonder about a bit of blue-green mold.

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7 other things to know this week ...

• Sunday September 28: "Open your newspaper any day of the week and you will find a report from somewhere in the world of someone being imprisoned, tortured or executed because his opinions or religion are unacceptable to his government." — Peter Benenson, founder of Amnesty International, in "The Forgotten Prisoners," the article that launched the organization on this day in 1961.​
• Monday September 29: "Dear friends, we have resumed the journey that we promised to continue for you; dear friends, your loss has meant that we could confidently begin anew; dear friends, your spirit and your dream are still alive in our hearts."— Commander Frederick Hauck, speaking from orbit aboard the space shuttle Discovery, which launched this date in 1988, 975 days after the Challenger disaster.
• Tuesday September 30: "I have not seen complete bibles but only a number of quires of various books [of the Bible]. The script is extremely neat and legible, not at all difficult to follow [You] would be able to read it without effort, and indeed without glasses." — Future Pope Pius II, describing pages from the Gutenberg Bible, which was completed on this day in 1454.
• Wednesday October 1: "It is my wish that it may not only be looked upon as a compilation of tried and tested recipes, but that it may awaken an interest through its condensed scientific knowledge which will lead to deeper thought and broader study of what to eat."— Fannie Farmer, from her famous cookbook, as she opened Miss Farmer's School of Cookery in Boston on September 24, 1902.
• Thursday October 2: "Everybody needs beauty ... places to play and pray in, where nature may heal and cheer and give strength to body and soul alike." — John Muir, whose advocacy led to Yosemite National Park being established on this day in 1890.
• Friday October 3:"Good ol' Charlie Brown" — First words spoken about Charlie Brown in the debut Peanuts comic strip on this day in 1950, beginning what would become the most successful comic strip in history.​
• Saturday October 4: "It is an hour of great joy. It is the end of many illusions. It is a farewell without tears." —East German Prime Minister Lothar de Maizière, speaking just before midnight on October 3, 1990, as East and West Germany reunited. Crowds sang the German national anthem, and the German flag was raised in front of the Reichstag building.