In 1880 Pasteur—who observed, “Chance favors the prepared mind”—was trying to prove he had isolated the cause of chicken cholera. He inoculated healthy chickens with the bacteria. They died. Then chance intervened. He had put aside a virulent culture for several days, then used it to inoculate more chickens. They lived. More significant, those same chickens survived when exposed to other virulent cultures. Crediting Jenner for the idea, he tried to weaken, or “attenuate,” his word, cultures and use them to immunize birds against lethal bacteria. He succeeded.
