In the later 1800s, the brilliant french chemist louis pasteur worked with chicken cholera, passing the disease agent from sick to healthy chickens. he suspended his research and went on vacation for several weeks. when he returned, the old cholera culture he gave to healthy chickens failed to kill them. he assumed his culture was bad and injected these same chickens with a fresh culture of chicken cholera. these chickens remained healthy while new chickens injected with the fresh culture of chicken cholera rapidly became sick and died. from our current knowledge of immunity, how would these observations be explained