1 6 CAUSE FOR VARIATION 



the course of the disease varies on the one side with the resist- 

 ance of the host and on the other with the degree of virulence 

 of the infecting microorganism. This has been expressed in 

 the formula 



in which D = the disease, V = the virulence of the infecting 

 organism, and R = the resistance of the host or the individual 

 attacked. As V or R change the disease is modified. For 

 example, rabbits that are partially immunized against swine 

 plague bacteria, when inoculated with a virulent culture of 

 that organism, will live for several days and perhaps for weeks, 

 and then die of peritonitis, pleuritis or extensive pus forma- 

 tions, instead of perishing within twenty-four hours with sep- 

 ticemia as they would if they had not been protected against 

 this organism. In chronic cases of swine plague, as found in 

 certain outbreaks, the bacteria are often attenuated so that 

 when inoculated into susceptible rabbits the result is the same 

 as when the rabbits protected by partial immunization were 

 inoculated with virulent cultures. The above simple formula 

 which was worked out and demonstrated for certain swine 

 diseases seems to apply to infectious diseases generally. 



§ 9. Classification or grouping of the infectious dis- 

 eases. It will be found in the study of the morbid anatomy 

 of the various specific maladies that the lesions in a given dis- 

 ease vary in different species and in individuals of the same 

 species to a marked degree. This fact precludes the possibility 

 of classifying or arranging them after their morbid anatomy, if 

 the idea of a specific etiology is to be adhered to. If the infec- 

 tious diseases are to be considered as parasitisms, as they 

 appear to be, the only logical method of classifying them, 

 according to the writer's opinion, is the one suggested by their 

 etiology, namely, that they shall be placed in groups corre- 

 sponding to their causes. Thus a single lesion found in the 

 glands of the head, in the lungs, in the liver, in the mesenteric 



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