22 CAUSE OF VARIATIONS 



§ 15. Cause for the variations in the course of an in- 

 fectious disease. It is a recognized fact that there is much 

 variation in the course of infectious diseases in different epizo- 

 otics and often marked individual variations occur in the same 

 outbreak. In explaining this interesting phenomenon, it is 

 important to take into account the question of individual resist- 

 ance or immunity — partial or more complete. It was found in 

 case of certain diseases that when an individual is partiallj' im- 

 munized and then infected that the lesions were very much 

 modified. The teachings of a specific etiology pointed to this 

 phenomenon as a result of certain biological or vital differences 

 existing either in the parasite or in the host, possibly in both. 

 The results of the investigations already made along this line, 

 suggest a probable explanation, that the course of the disease 

 varies on the one side with the resistance of the host and on 

 the other with the degree of virulence of the infecting micro- 

 organism. This has been expressed in the formula 



r 



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 ex- 

 ample, 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 formations 

 (Fig. 3) instead of perishing within twenty-four hours with 

 septicaemia 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 end is the same as 

 when the rabbits protected by partial immunization were in- 

 oculated with virulent cultures. The above simple formula 

 which was worked out and demonstrated for certain swine dis- 

 eases seems to apply to infectious diseases generally. 



§ 16. Grouping of the specific infectious diseases. 

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



