42 Manual of Veterinary Microbiology. 



ism to such a degree that the disease quickly dies out 

 if their original virulence is not restored by a return 

 to the outer media ; it is thus with cholera, for exam- 

 ple, which spontaneously disappears in winter in the 

 countries of central Europe, probably because the ex- 

 ternal media have become unsuitable to its multipli- 

 cation. In the case of cholera we perceive a grada- 

 tion toward the miasms ; but whilst in this last the 

 microbe exhausts its effects upon the individual which 

 harbors it and does not extend beyond it, the bacillus 

 of cholera, on the contrary, proliferates during its pass- 

 age through the organism of man and thus increases 

 the chances of later infection. 



The source of infection, therefore, for germs of this 

 kind is twofold : the diseased organism and the in- 

 fected media in which the germs pullulate; these two 

 sources possess in the same degree the power of be- 

 getting the disease, and the opportunities of infection 

 will be more frequent than for the germs of the first 

 group in as much as the multiplication of the microbes 

 in the different media is an important cause of their 

 preservation. 



Non-contagious facultative imrasites. — These live, in 

 the normal condition, in the external media, and it is 

 only incidentally that they develop within the organ- 

 ism of animals. They do not seem in the latter to 

 meet with conditions favorable to their vitality, for 

 their effects are exhausted completely in the course of 

 the disease which they determine, and the disease is 

 not transmitted from one subject to another. The 

 condition under which it occurs consists always in the 

 impregnation of a healthy organism by germs drawn 

 directly from external infected media ; in short, the 



