NATURAL IMMUNITY 65 



of which depend upon subjecting the germs to preju- 

 dicial influences of one kind or another long exposure 

 to oxygen, exposure to a temperature a little short of 

 that which would completely destroy their vitality, 

 exposure to various chemical agents. 



Attenuated germs may cause infection in very sus- 

 ceptible animals, and may gain in virulence as a re- 

 sult of their growth in such animals. After passing 

 through a series of susceptible animals they may 

 finally acquire such pathogenic virulence that they 

 can overcome the resisting power of animals having 

 a considerable degree of natural immunity. Apply- 

 ing the facts ascertained by experiments upon the 

 lower animals, we can understand how the earlier 

 cases in an epidemic may occur in the most suscept- 

 ible individuals, and are often comparatively mild ; 

 but, as a result of its transmission through a series 

 of individuals, the germ gradually increases in vir- 

 ulence and the epidemic in malignancy. Thus the 

 earlier cases in an epidemic of diphtheria or of scarlet 

 fever are often mild, while later cases prove to be 

 extremely difficult to manage and show a high rate of 

 mortality. 



Infection also depends to some extent upon the 

 number of germs introduced. The resources of na- 

 ture, upon which immunity depends, may be suffi- 

 cient to dispose of a few typhoid bacilli or diphtheria 



