PARASITIC BACTERIA. 143 



measles, whooping-cough, peritonitis, pneumonia, 

 syphilis. 



Still another list might be given of diseases 

 whose general nature indicates that they are 

 caused by bacteria, but in connection with which 

 no distinct bacterium has yet been found. As 

 might be expected also, a larger list of animal 

 diseases has been demonstrated to be caused by 

 these organisms. In addition, quite a number 

 of species of bacteria have been found in such 

 material as faeces, putrefying blood, etc., which 

 have been shown by experiment to be capable of 

 producing diseases in animals, but in regard to 

 which we have no evidence that they ever do 

 produce actual disease under any normal con- 

 ditions. These may contribute, perhaps, to the 

 troubles arising from poisonous foods, but can 

 not be regarded as disease germs proper. 



VARIABILITY OF PATHOGENIC POWERS. 



As has already been stated, our ideas of the 

 relation of bacteria to disease have undergone 

 quite a change since they were first formulated, 

 and we recognise other factors influencing dis- 

 ease besides the actual presence of the bac- 

 terium. These we may briefly consider under 

 two heads, viz., variation in the bacterium, and 

 variation in the susceptibility of the individual. 

 The first will require only a brief consideration. 



That the same species of pathogenic bacteria 

 at different times varies in its powers to produce 

 disease has long been known. Various con- 

 ditions are known to affect thus the virulence of 

 bacteria. The bacillus which is supposed to give 

 rise to pneumonia loses its power to produce the 

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