lo INFECTIOUS AND PARASITIC DISEASES. 



Infectious diseases are divided into the 

 Specific and specific and the non-specific according 

 Non-specific as they result from a specific or non- 

 Infectious specific agent. There are some infectious 

 Diseases. diseases, however, in which the etiological 



factor is not known, yet they are classed 

 with the specific maladies. This is done because the 

 symptomatology of each is characteristic, and because 

 any one of them in one individual never gives rise in 

 another, who may contract it, to a different disease. 

 In this class of specific diseases, of which the causes 

 are not known, is small-pox, scarlet fever, measles, 

 chicken-pox, etc. These same diseases are also classed 

 as infectious, but entirely, it should be noted, on account 

 of their close resemblance to those infectious diseases 

 whose determining factor is positively known to be a 

 micro-organism. 



In the list of infectious diseases of known and unknown 

 etiology which follows, it will perhaps come as a sur- 

 prise to the student that the cause of so many diseases 

 is as yet unknown ; and that those of unknown etiology' 

 include many of our most common diseases. He 

 might well ask the question, why have the causes of 

 these every-day diseases not been discovered? There 

 are probably two chief reasons : First, because the causes 

 have been sought for as if they must be bacteria, 

 whereas they very likely belong to entirely different 

 genuses of organisms; and second, because the causes 



