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?
Inoculation with an old, weakened strain confers resistance to disease.
Vibrio cholera only infects old chickens.
Vibrio cholera is a pathogen.
Inoculation with Vibrio cholera always leads to disease.