120 MICROBES AND TOXINS 



from which the spore was derived. Pasteur had obtained an 

 attenuation fixed by the resistant form, the spore ; " vaccine 

 viruses fixed in their germs, with all their peculiar qualities 

 without any possible alteration." 



It was by these modifications of virulence that Pasteur 

 explained the behaviour of the great epidemic diseases : 



" There exist virulent diseases which appear spontaneously in 

 every country : such is, for example, the typhus fever of armies 

 in the field. Without doubt the germs of the microbes 

 responsible for these maladies are to be found everywhere. 

 Man may carry them on his body or in his alimentary canal 

 without suffering great harm, but they are, nevertheless, ready 

 to become dangerous when, under conditions of overcrowding 

 and successive development on the surface of wounds, in 

 weakened bodies or otherwise, their virulence becomes progres- 

 sively reinforced. 



" Virulence thus appears under a new light to us and one 

 which is distinctly disquieting for mankind, unless nature during 

 the past centuries has already met with all the opportunities 

 possible of developing virulent or contagious diseases, which 

 is highly improbable. 



"A microscopical organism, harmless for man or for a given . 

 animal species, is simply a creature which cannot develop in 

 our bodies or in the body of the given animal, but there is 

 nothing to prove that if this microscopical creature succeeded 

 in penetrating another of the many thousand species in creation, 

 it might not invade it and produce in it disease. Its virulence, 

 reinforced then by successive passages through individuals of 

 this species, might become powerful enough to attack such 

 and such an animal of higher position, man, or the domestic 

 animals. In this way it is possible to create new virulences 

 and new contagions. I am much inchned to think it is 

 thus that there have appeared throughout the ages small-pox, 

 syphilis, plague, yellow fever, etc. . . . and further that it is 

 by phenomena of this kind that there from time to time 

 appear certain great epidemics. ..." 



