536 MICRO-ORGANISMS AND DISEASE [chap. 



lose their specific action altogether. Examples illustrating 

 this have been mentioned on various previous occasions 

 and they are familiar to every bacteriologist : the attenuated 

 anthrax vaccines obtained by Pasteur by growing bacillus 

 anthracis in chicken broth at 42-5° C, and successfully used 

 for protective inoculation, the attenuation of the bacillus of 

 fowl cholera by Pasteur, of the bacillus tuberculosis grown 

 on Glycerin Agar, of the pneumonococcus, of the bacillus 

 of malignant oedema, of streptococcus of erysipelas, and 

 many other microbes. Similarly it is a common experience 

 that a specific microbe which possesses a low virulence, or 

 which has altered or lost its specific pathogenic action, can 

 by altering the soil (artificial medium or animal body) 

 become more virulent or recover its former virulence respec- 

 tively. All these phenomena are constantly met with in all 

 bacteriological work, and very few microbes are exempt from 

 such changes. 



Now, the questions that to the sanitarian are of great 

 importance are these: (i) can a parasitic microbe which 

 although at first derived from a virulent animal source, but 

 existing under abnormal conditions inside or outside the 

 animal body, alter its physiological nature so as to cease to 

 be any longer capable of being a pathogenic or parasitic 

 microbe? (2) can such a degraded microbe, i.e. once patho- 

 genic but now hving as a saprophyte again, under altered con- 

 ditions resume its virulence? and (3) can a true saprophyte, 

 that is a microbe not at any time connected with patho- 

 genicity, owing to certain peculiar conditions under which it 

 has been living when introduced into the animal assume the 

 nature of a parasite ? 



From what has been stated previously, laboratory experi- 

 ence justifies us in answering questions (i) and (2) in the 

 affirmative, but it is more difficult to give a decided 

 answer to question (3), for it is quite possible to imagine, it 



