ber of the same
year Pasteur announced the method by which this "attenuation of the
virus," as he termed it, had been brought about--by cultivation of the
disease germs in artificial media, exposed to the air, and he did not
hesitate to assert his belief that the method would prove "susceptible
of generalization"--that is to say, of application to other diseases
than the particular one in question.

Within a few months he made good this prophecy, for in February,
1881, he announced to the Academy that with the aid, as before, of his
associates MM. Chamberland and Roux, he had produced an attenuated virus
of the anthrax microbe by the use of which, as he affirmed with great
confidence, he could protect sheep, and presumably cattle, against that
fatal malady. "In some recent publications," said Pasteur, "I announced
the first case of the attenuation of a virus by experimental methods
only. Formed of a special microbe of an extreme minuteness, this virus
may be multiplied by artificial culture outside the animal body. These
cultures, left alone without any possible external contamination,
undergo, in the course of time, modifications of their virulency to a
greater or less extent. The oxygen of the atmosphere is said to be
the chief cause of these attenuations--that is, this lessening of the
facilities of multiplication of the microbe; for it is evident that the
difference of virulence is in some way associated with differences of
development in the parasitic economy.

"There is no need to insist upon the interesting character of these
results and the deductions to be made therefrom. To seek to lessen the
virulence by rational means would be to establish, upon an experimental
basis, the hope of preparing from an active virus, easily cultivated
either in the human or animal body, a vaccine-virus of restrained
development capable of preventing the fatal effects of the former.
Therefore, we have applied all our energies to investigate the possible
generalizing action of atmospheric oxygen in the attenuation of virus.

"The anthrax virus, being one that has been most carefully studied,
seemed to be the first that should attract our attention. Every time,
however, we encountered a difficulty. Between the microbe of chicken
cholera and the microbe of anthrax there exists an essential difference
which does not allow the new experiment to be verified by the old.
The microbes of chicken cholera do not, in effect, seem to resolve