In the late-summer heat of 1881, in the Scottish village of Darvel, not far from Loudon Hill, where William Wallace defeated the English in the Wars of Scottish Independence, a boy was born to the farmer Hugh Fleming and his second wife, Grace, the daughter of a neighboring farmer, whom Hugh married when he was 59 and she was 27.

The boy, Alexander, was a quick learner. He attended the town school, where he earned a two-year scholarship to Kilmarnock Academy, the only school in Scotland that would educate two Nobel Prize laureates, thus tying the supposedly more prestigious Eton College, in England. Then Alexander was off to London, where he attended the Royal Polytechnic, founded as “an institution where the public, at little expense, may acquire practical knowledge of the various arts and branches of science connected with manufacturers, mining operations and rural economy.”

In 1903, following the path of his older brother Tom, Alexander stayed on in the city and enrolled at St. Mary’s Hospital Medical School, where he received a medical degree. He continued at St. Mary’s in the research department, becoming an assistant bacteriologist under Almroth Wright, who had worked with the British military to develop vaccines and immunities in soldiers and sailors before setting up shop at the hospital.

Wright was a formidable mentor. He was correct that many soldiers died from preventable diseases, and he helped to prepare vaccines for British troops in World War I. He foresaw that antibiotics would create resistant bacteria. But Wright also thought microorganisms do not cause disease (for which he was later dubbed Almroth Wrong) and, after he had been knighted, he campaigned vigorously against women’s suffrage, using his prestige as a scientist, on the grounds that women’s brains were not suited to deal with public issues or any profession. Perhaps this is only surprising if one subscribes to the idea that there are geniuses of all trades rather than science being the result of many imperfect people fumbling in the dark.

Alexander, meanwhile, was commissioned first a lieutenant and then a captain in the Royal Army Medical Corps, serving in hospitals and aid stations in France throughout World War I. It was then he observed that antiseptics did not prevent soldiers from dying of infected wounds; they contributed to their deaths.

Back at St. Mary’s, he began a search for a better antibacterial agent. In time, he took up the study of Staphylococcus. He had cultures growing in his laboratory (which is preserved today as a museum) in 1928 when he left on vacation with his wife, an Irish nurse, and their toddler, Robert. When he returned, several weeks later, he looked at his molds in the lab and said, “That’s funny.” A fungus was destroying his bacteria.

“One sometimes finds what one is not looking for,” he explained much later. “When I woke up just after dawn on September 28, 1928, I certainly didn’t plan to revolutionize all medicine by discovering the world’s first antibiotic ... But I suppose that was exactly what I did.”

Alexander Fleming had stumbled on penicillin. He didn’t think much of it at first, however, and by the later 1930s, he had left off studying it.

Other researchers figured out how to mass produce it, and, with backing from the British and American governments, by 1944, enough penicillin was made to successfully treat all the Allied wounded in World War II. Marketed as a prescription drug after the war, it helped defeat gangrene and tuberculosis. In 1945, Fleming won the Nobel Prize. The age of modern antibiotics, an age of miracles, had begun.