Vibrio cholera is a bacterium that causes the diarrheal disease known as cholera. In the 1800s
Louis Pasteur noticed that old Vibrio cholera cells did not cause disease in chickens, whereas new
V. cholera cells did cause disease. He also noticed that chickens previously inoculated with old
cells could not be made sick by new V. cholera cells. Based on Pasteur's observations, which of the
following represents a good hypothesis?