4 
274 PROTECTIVE INOCULATIONS. 
ulence may be reproduced by cultivation. Finally, as shown in one of our 
recent communications, since one attack of anthrax protects, each one of our 
attenuated microbes of charbon constitutes a vaccine for the microbe of 
superior virulence ; that is to say, a virus suitable to produce a more benign 
malady. What, then, is more easy than to find among these a virus suitable 
to give anthrax to sheep, cows, or horses, without causing them to perish, 
and capable of preserving them from a subsequent fatal attack? We have 
already practised this operation upon sheep with great success.” 
At the end of this important communication Pasteur says: 
“‘T concluded my communication of October 26th by remarking that the 
attenuation of virus by the influence of the air is probably one of the factors 
in the extinction of great epidemics. The facts just recorded, in their turn, 
may serve to explain the so-called spontaneous appearance of these scourges. 
An epidemic which has been terminated by the attenuation of its virus may 
be relighted by the reinforcement of this virus under certain influences. The 
accounts which I have read of the spontaneous appearance of the plague ap- 
pear to me to offer examples of this. The plague is a virulent malady which 
prevails in certain countries. In all of these countries its attenuated virus 
probably exists, ready to take its active form when the necessary conditions 
as to climate, famine, and distress again prevail. There are other virulent 
maladies which appear spontaneously in all countries, such as camp typhus. 
Without doubt the germs of the microbes which cause these.diseases are 
everywhere distributed. Man carries them about him, or in his intestine, 
without great damage, but ready, nevertheless, to become dangerous when, 
as a result of certain conditions or of successive development upon the sur- 
face of wounds, in bodies enfeebled or otherwise, their virulence is progres- 
sively reinforced: And from this point of view virulence appears to us 
under a new light which is somewhat disquieting for humanity, unless na- 
ture, in the evolution which has occurred during the past centuries, has al- 
ready encountered all possible occasions for the production of virulent or 
contagious diseases, an assumption which seems very improbable. 
‘What is an inoffensive microscopic organism for man or for a given 
animal? It is an organism which cannot develop in our body or in that of 
the animal; but nothing proves that if this microscopic organism should 
penetrate into some other of the thousands of species of the creation, it could 
not invade it and cause it to become sick. Its virulence, then, reinforced by 
passing through a series of individuals of this species, might become such 
that it could invade man or one of the domestic animals. By this means new 
contagions may be created. I am disposed to believe that it is in this way 
that, in the course of ages, have appeared small-pox, syphilis, the plague, 
yellow fever, etc.” 
This broad induction has received considerable support from more 
recent researches, which show that the typhoid bacillus, the cholera 
spirillum, and other important pathogenic bacteria become attenuated 
when they lead a saprophytic existence for some time, and regain their 
virulence when they are propagated within the bodies of susceptible 
animals. 
In a later communication (March 21st, 1881) Pasteur says that he 
has found by experiment that when attenuated varieties of the anthrax 
bacillus form spores, these again reproduce the same pathogenic va- 
riety, so that cultures of each degree of attenuation can be maintained . 
indefinitely. 
