254 BACTERIA IN INFECTIOUS DISEASES. 



Pasteur asserts that this virulence may be restored 

 by inoculating guinea-pigs but a day old, which 

 succumb to this attenuated virus, although those 

 which are five or six days old are proof agains it. 

 After several successive inoculations, older guinea- 

 pigs are killed, and after a time the virus becomes 

 sufficiently potent to destroy a full-grown animal. 

 Finally it regains its full activity, and will kill a 

 sheep. 



The form of induced septicaemia in the rabbit, 

 which has been especially studied by the writer 

 (see p. 355), furnishes a good example of an in- 

 fectious disease resulting in one species of animal 

 from the introduction into its body of a micro- 

 organism which is harmless for other species. 

 This organism a micrococcus is commonly 

 found in normal human saliva, where it is asso- 

 ciated with various other species. Experiments 

 thus far made indicate that there are various 

 physiological varieties (breeds) of this micrococ- 

 cus, varying in pathogenic power ; for the saliva 

 of different individuals differs in virulence. This 

 may be accounted for by the fact that the con- 

 ditions are not identical. The human mouth is a 

 culture-apparatus in which the conditions are ex- 

 tremely favorable for the development of these 

 minute plants; the secretions from the salivary 

 glands afford a constant supply of pabulum, and 

 the temperature is maintained at a fixed point. 

 But the flow of saliva is more abundant in some 

 persons than in others; and the presence of de- 



