162 
M. L. PASTEDR. 
tiplied indefinitely in a few hours, without necessitating the use of more carbun- 
culous blood. 
The preceding facts present a problem of great interest. I mean to speak of 
the possible return of the virulency of attenuated or even dead viruses. We, for 
instance, have just obtained a carbuncular bacteridie free from all virulency for 
the cobaye, rabbit and sheep. Can we return to it its activity in relation to 
these species of animals? We have also prepared the microbe of chicken 
cholera, deprived of all virulency for chickens. How can we return to it the 
power of development in these Gallinaceous ? 
The secret of those returns to virulency remains altogether, actually, in suc¬ 
cessive cultivation in the bodies of certain animals. 
Our bacteridie, harmless for the cobayes, is not so at all ages of those ani¬ 
mals ; but how short is the period of virulency! A cobaye several years old, one 
year, six months, one month, two or three weeks, eight, seven, six days, or even 
less, runs no danger of disease or death by the inoculation of the bacteridie we 
are speaking of ; but, on the contrary, it kills the cobaye of one day. Surprising 
result! Our experiments on this point have never failed. If, then, one passes 
from the cobaye of one day to that of another, by the inoculation of the blood of 
the first to that of the second, from this last to a third, and so on, progressively, 
the virulency of the bacteridie will increase; in other words, it accustoms itself 
to development in the economy. Soon one may kill cobayes three or four days 
old, one week, one month, or several years of age, even sheep themselves. It has 
regained its original virulency. Without hesitation, though we have not as yet 
had occasion to test it, we may say that it would kill cows and horses, as it pre¬ 
serves this condition indefinitely if nothing is done to attenuate it again. 
As it concerns the microbe of chicken cholera, when it has lost its power 
of action upon chickens its virulency can be returned to it through small birds— 
all species that it kills at once. Then, by successive journeys in the bodies of 
those animals, it may gain, by degrees, a virulency which will, de novo, manifest 
itself upon adult chickens. 
Is it necessary to add that, in this return to virulency, and between its pro¬ 
gressive steps, vaccine virus can be prepared of all degrees of virulency for the 
bacteridie, and that it is the same for the microbe of cholera ? 
This question of the return to virulency is of the greatest interest in the 
etiology of contagious diseases. 
I closed one of my last communications in remarking that the attenuation of 
viruses by the influence of the air must be a factor of the extinction of great epi¬ 
demics. The preceding facts, in their turn, may serve to explain the so-called 
spontaneous appearance of these scourges. An epidemy which has died out by the 
attenuation of its virus may return by the strengthening of this virus under cer¬ 
tain influences. What I have read of the spontaneous appearance of the plague 
seems to be examples of it, such as the plague of Benghazi in 1856-’58, whose 
appearance could not be traced to any original contagion. Plague is a virulent 
disease proper to some countries. In those its attenuated virus must exist, 
ready to resume its active form when conditions of climate, famine, misery, 
exist again. There are other diseases which appear spontaneously in all coun¬ 
tries—such as typhus of armies, no doubt the germ of the microbe, the authors of 
