CHAPTER I. 



GENERAL CONSIDERATION OP ETIOLOGY, INFECTION 

 AND SPECIFIC INFECTIOUS DISEASES. 



§ I. Etiology. The development of the science of bac- 

 teriology, together with a knowledge of the parasitic protozoa^ 

 has already demonstrated that a large number of the infectious 

 diseases are the direct result of the invasion of the animal body 

 by certain species of microorganisms. A specific etiology 

 which teaches that for each of the various epizootics we have a 

 single, definite cause has become recognized and been accepted 

 by all pathologists. Although there are a number of distinct 

 diseases for which such a specific agent has not been found, 

 the evidence in the very nature of the maladies is conclusive 

 that for each of these such an etiological factor exists. 



In studying the pathology of infectious diseases the idea 

 of a definite and adequate cause should be kept in mind. It is 

 no longer justifiable to attribute them to an unfavorable en- 

 vironment, poor hygiene, or improper sanitation, conditions 

 which may aid the specific cause but which cannot supplant 

 it. It is often a troublesome task to differentiate between the 

 morbid affections, often fatal in their results, brought about by 

 improper care and food, and the maladies dependent upon a 

 specific cause. The reason why many of the former theories 

 accepted unsanitary conditions, certain kinds of food or other 

 similar agencies as the etiology of distinct, infectious diseases, 

 is found in the fact that the infectious microorganisms were 

 and still are often transmitted to the individual through such 

 channels. It is important, therefore, that the limitations of 

 both the exciting cause and the environment should be fully 

 taken into account. Although for certain diseases such as 

 rinderpest we do not know just what the specific cause is, 



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