Before 1922, a diagnosis of type 1 diabetes was, for most children, a sentence of death.
Their bodies could not use the sugar in their blood, and the only treatment doctors could offer was a diet so strict that it bought most patients only a year or two and left them painfully weak.
That changed with remarkable speed, thanks to work done at the University of Toronto by a young surgeon and a student who had barely met.
The surgeon was Frederick Banting, a farmer's son whose medical practice in London, Ontario, was struggling.
In October 1920, while preparing a lecture, he read an article about the pancreas and, according to a story he often told, woke in the middle of the night to write down an idea.
Scientists already suspected that small groups of cells in the pancreas produced a substance that controlled blood sugar, but every attempt to extract it had failed.
Banting guessed that digestive juices from the rest of the organ were destroying it, and he proposed a way around the problem.
He took his idea to John Macleod, a respected professor of physiology in Toronto.
Macleod was doubtful, since Banting had little research experience, but agreed to lend him laboratory space for the summer of 1921, along with dogs for his experiments and a student assistant.
That assistant was Charles Best, who was just twenty-two and who, according to a well-known story, won the job on the toss of a coin.
The work was slow: through the summer heat, the two men tested extract after extract on dogs with diabetes, with results that were often confusing or simply wrong.
By the fall, however, they had evidence that their extract could lower blood sugar, and Macleod, back from Scotland, put his department's resources behind the project.
A talented biochemist, James Collip, joined the team to purify the extract, which was still too crude to be safe for humans.
In January 1922, Leonard Thompson, a fourteen-year-old boy who weighed only sixty-five pounds, became the first patient to receive it at Toronto General Hospital.
The first injection achieved little.
Twelve days later he was given a purer version prepared by Collip, and this time the effect was dramatic: his blood sugar fell toward normal, his strength began to return, and he went on to live for another thirteen years.
Success brought its own tensions.
Banting, who distrusted Macleod and felt the professor had claimed too much credit, was furious when the Nobel Prize of 1923 went to the two of them and not to Best.
He announced that he would share his prize money with his young assistant, and Macleod responded by sharing his with Collip.
Arguments over who had done what went on for decades, and historians still disagree about each man's share.
On one question, however, the team showed remarkable generosity.
Banting believed that a doctor should not profit from a treatment that could save lives, and at first he did not even want his name on the patent.
In the end, he, Best and Collip sold their rights to the University of Toronto for one dollar each, so that the university could control the drug's quality and license companies to make it without the inventors growing rich.
Words often attributed to Banting sum up the spirit of the decision: insulin, he is supposed to have said, belonged not to him but to the world.
A century later, the one-dollar sale is still one of the best-loved stories in medicine, a reminder that a discovery's value is not measured only by what it earns.