On January 11, 1922, 14-year-old Leonard Thompson became the first person to receive an injection of the hormone insulin for Type-1 diabetes—a disease that for millennia had been considered a death sentence for anyone who developed it. The breakthrough would be one of the most consequential in medical history, saving millions of lives.

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.