CH. VII] VARIATIONS IN PATHOGENICITY 97 



lesions to which they give rise, is no greater than the contrast 

 existing between different strains of Koch's bacillus in the 

 same respect ; if we ascribe the contrast in the former 

 case to specific diflFerences in pathogenicity one is forced to 

 ascribe it in the latter case to the same factor and acknowledge 

 that different strains of the tubercle bacillus exhibit marked 

 differences in pathogenicity. 



There is one not unlikely fallacy which needs to be 

 guarded against before we can with confidence attribute to 

 an organism any unusual symptoms which appear to follow 

 its invasion of the body. A certain disease may be latent in 

 the patient, that is to say present without giving rise to any 

 noticeable symptoms. The constitutional disturbances arising 

 from infection by the organism in question may " light up " 

 this pre-existing disease and the symptoms of the latter may 

 then be incorrectly credited to the invading organism. 



Such a sequence is well illustrated by a case recorded by 

 Roberts and Ford under the title " A case of Cerebrospinal 

 Fever simulating Acute Nephritis with uraemic convulsions.'' 

 The patient suffered from typical symptoms of acute nephritis 

 and uraemia — dropsy, the passage of scanty urine loaded with 

 albumen, convulsions and coma — and showed marked im- 

 provement as a result of the treatment usually adopted for 

 these conditions. The meningococcus was, however, isolated 

 from the spinal fluid and the symptoms on this account 

 attributed to that organism. In this case a pre-existing 

 nephritis might, conceivably, have given rise at the onset of 

 the illness to the symptoms characteristic of that disease 

 before those characteristic of cerebrospinal fever had had 

 time to develop ; or the symptoms of the first disease might 

 have completely masked those of the second. 



2. In the second place, one and the same strain of an 

 orga/nism can by a/rtifieial means he so modified as to cause 

 an altogether different type of disease. For example, Madame 

 Henri (1914) found that the pathogenicity of B. anthracis 

 was changed to a remarkable degree by exposure to the 

 ultra-violet rays. Its subsequent injection into an animal 

 produced symptoms quite unlike those caused by the normal 



D. 7 



