PATHOLOGICAL PHYSIOLOGY. 
25 
following conclusions : 1st, Epizootic cholera, even the most vir¬ 
ulent, can be prevented by inoculations of attenuated virulent 
virus. 2d, The duration of immunity extends beyond twelve 
months, and is sufficient for the requirements of the raising of 
swine, as the period occupied in the fattening of the animals does 
not usually extend beyond one year. 
However, added Mr. Pasteur, notwithstanding these happy 
results, I regret that the question of the appropriation of vaccines 
to various breeds requires yet new observations, before vaccina¬ 
tion can become generalized. In waiting for definite results, 1 
desire to make known at present the method which we have used 
to attenuate the virus of cholera. 
The observations made in my laboratory have established the 
fact that viruses are morbid entities, but that they may affect 
forms and also physiological properties according to the media 
in which they live and multiply, and consequently, though viru¬ 
lency belongs to microscopical living species, it is essentially sus¬ 
ceptible to modifications. It may be attenuated or increased, and 
each of these conditions is capable of being fixed by culture. 
A microbe is virulent for an animal when it has the faculty of 
multiplying in its body, like a parasite, and of giving rise, while 
reproducing itself, to disorders which may result in a fatal issue. 
If, for example, the microbe which has lived in an animal of any 
given species has passed out of one individual of that species to 
penetrate into another of the same nature, without having in the 
interval changed by any sensible external influence, the virulency 
of this parasite can be considered, so to speak, to have acquired 
a fixed maximum potency for the individuals of that breed. The 
parasite of anthrax, for instance, which is special to sheep, va¬ 
ries but little from one subject to the other and from one year to 
another for a given country. This may no doubt be accounted 
for by the hypothesis that through the various transmissions from 
sheep to sheep, the parasite has become accustomed to live in 
sheep in a state that may be called definite. But the virulency 
that has not yet reached its maximum of power may be essen¬ 
tially modified by its passage through a series of individuals of 
the same breed, and I repeat that when we have tried to endue 
