CHAPTER XIV. 



CONTAGIOX AND CONTAGIOUS DISEASES. 



The origin of contagious diseases has been, and is yet to some 

 extent, a matter of controversy. It is, however, generally 

 accepted that they are caused by specific pathogenic micro- 

 organisms, having the power of inducing specific morbid lesions 

 when introduced into the animal body. 



Many writers assert that a contagious disease can only arise 

 from the introduction of a specific microbe existing in the fluids 

 and solids of a diseased animal into the body of another, and 

 Koch laid down the following postulates, to which it was 

 supposed there could be no exceptions; but further observa- 

 tions have proved that there are many exceptions to his con- 

 clusions, and an explanation is thus given to the belief in a 

 spontaneous origin of a contagious disease. Koch's postulates 

 are as follows : — 



1. The micro-organism must be found in the blood or 

 diseased tissue of men or animals suffering from or dead of 

 the disease. 



2. These micro-organisms must be isolated from the blood, 

 lymph, or tissues, and cultivated in suitable media outside the 

 animal body. To obtain these cultivations pure they must 

 be carried on through successive generations of the organism. 



3. A pure cultivation thus obtained must, when introduced 

 into the body of a healthy animal, produce the disease in 

 question. 



4. Lastly, in the inoculated animal the same micro-organism 

 must again be found. 



Although coming from such a high authority, these postulates 

 must not be accepted in their entirety, for we have many microbic 

 diseases where this form of origin cannot possibly have an 



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