While studying a particular strain of cholera in 1879, he and his assistant Charles Chamberland forgot to infect a brood of lab chickens with the disease before they headed out on vacation (Pasteur and Chamberland, not the chickens). Upon returning to Paris weeks later, Pasteur injected the birds with the now weakened culture and found that instead of dying, they became immunized. From this research Pasteur invented several lifesaving vaccines.
