Louis Pasteur Inoculates Chickens Against Cholera
During the heat of the summer, Pasteur returned to Paris leaving the cholera cultures used for infection stored on the shelves of the Arbois laboratory.
Upon return, Pasteur's collaborators were disappointed to find that these stored cultures no longer killed injected chickens, nor even made them sick. The group set to work to make new cultures of the bacillus and tested these batches on new birds and those healthy previously treated birds. The results were astonishing: The previously injected birds were unaffected by the bacillus, while the new birds all died.
Just as each kind of fermentation possesses a definite organized ferment, so many diseases are dependent on the presence of a distinct microbe; and just as the gardener can pick out and grow a given plant or vegetable, so the bacteriologist can (in many cases) eliminate the adventitious and grow the special organism -- in other words, can obtain a pure cultivation which has the power of bringing about the special disease. But by a process of successive and continued artificial cultures under different conditions, the virus of the organism is found to become attenuated; and when this weakened virus is administered, the animal is rendered immune against further attacks. The first disease investigated by Pasteur was that of chicken cholera, an epidemic which destroyed 10% of the French fowls; after the application of the preventive method the death rate was reduced to below 1%.