A mother begged a scientist to inject her dying son with something that had never been tested on a human being. It was July 1885 in Paris. Nine-year-old Joseph Meister stood trembling in Louis Pasteur's laboratory, his small hands and legs covered in deep bite wounds. Two days earlier, a rabid dog had attacked him in his village. The animal had been killed immediately afterward, and it was confirmed to have rabies. His mother knew exactly what that meant. In 1885, rabies was a death sentence. Once symptoms appeared—the terror of water, the violent convulsions, the hallucinations—no one survived. Not ever. The death was agonizing, sometimes lasting days, and there was nothing anyone could do but watch. But she had heard whispers about a chemist in Paris. A man named Louis Pasteur who had been experimenting with something that might help. She didn't know if the rumors were true. She only knew her son was going to die unless she tried. "Please," she said to Pasteur. "Save my son." Louis Pasteur was 62 years old and already one of the most celebrated scientists in Europe. His discoveries had transformed industries and changed how we understand the world. But he faced an impossible decision. He did have a vaccine for rabies. He had spent years developing it, testing it successfully on animals again and again. But it had never been given to a human being. Pasteur wasn't even a medical doctor—he was a chemist. If he injected this boy with an experimental treatment and the child died, Pasteur could be charged with murder. His career, his legacy, everything he had built could be destroyed. But if he did nothing, young Joseph would certainly die. Pasteur consulted with two physicians who examined the boy. Their conclusion was unanimous: without treatment, there was no hope. The vaccine was his only chance. Pasteur made his decision. They would try.