Louis Pasteur produced the first laboratory-developed vaccine: the vaccine for chicken cholera (Pasteurella multocida).
Pasteur attenuated, or weakened, the bacteria for use in the vaccine. He happened upon the method of attenuation by accident: in his lab, he was studying fowl cholera by injecting chickens with the live bacteria and recording the fatal progression of the illness. He had instructed an assistant to inject the chickens with a fresh culture of the bacteria before a holiday. The assistant, however, forgot. When the assistant returned a month later, he carried out Pasteur’s wishes. The chickens, while showing mild signs of the disease, survived. When they were healthy again, Pasteur injected them with fresh bacteria. The chickens did not become ill. Pasteur eventually reasoned the factor that made the bacteria less deadly was exposure to oxygen.