At the University of Toronto, Canadian scientists Frederick Banting and Charles Best successfully isolate insulin—a hormone they believe could prevent diabetes—for the first time. Within a year, the first human sufferers of diabetes were receiving insulin treatments, and countless lives were saved from what was previously regarded as a fatal disease.

Diabetes has been recognized as a distinct medical condition for more than 3,000 years, but its exact cause was a mystery until the 20th century. By the early 1920s, many researchers strongly suspected that diabetes was caused by a malfunction in the digestive system related to the pancreas gland, a small organ that sits near the liver. At that time, the only way to treat the fatal disease was through a diet low in carbohydrates and sugar and high in fat and protein. Instead of dying shortly after diagnosis, this diet allowed diabetics to live—for about a year.

A breakthrough came at the University of Toronto in the summer of 1921, when Canadians Frederick Banting and Charles Best successfully isolated insulin from canine test subjects, produced diabetic symptoms in the animals, and then began a program of insulin injections that returned the dogs to normalcy. On November 14, the discovery was announced to the world.

Two months later, with the support of J.J.R. MacLeod of the University of Toronto, the two scientists began preparations for an insulin treatment of a human subject. Enlisting the aid of biochemist J.B. Collip, they were able to extract a reasonably pure formula of insulin from the pancreases of cattle from slaughterhouses. On January 23, 1922, they began treating 14-year-old Leonard Thompson with insulin injections. The diabetic teenager improved dramatically, and the University of Toronto immediately gave pharmaceutical companies license to produce insulin, free of royalties. By 1923, insulin had become widely available, and Banting and Macleod were awarded the Nobel Prize in medicine.