PARASITIC BACTERIA. 145 



only a simple abscess, or even to a type that is 

 entirely innocuous. It is this factor, doubtless, 

 which in a large measure determines the severity 

 of any epidemic of a bacterial contagious dis- 

 ease. 



SUSCEPTIBILITY OF THE INDIVIDUAL. 



The very great modification of our early 

 views has affected our ideas as to the power 

 which individuals have of resisting the invasion 

 of pathogenic bacteria. It has from the first 

 been understood that some individuals are more 

 susceptible to disease than others, and in attempt- 

 ing to determine the significance of this fact 

 many valuable and interesting discoveries have 

 been made. After the exposure to the disease 

 there follows a period of some length in which 

 there are no discernible effects. This is followed 

 by the onset of the disease and its development 

 to a crisis, and, if this be passed, by a recovery. 

 The general course of a germ disease is divided 

 into three stages: the stage of incubation, the 

 development of the disease, and the recovery. The 

 susceptibility of the body to a disease may be 

 best considered under the three heads of Inva- 

 sion, Resistance, Recovery. 



Means of Invasion. In order that a germ dis- 

 ease should arise in an individual, it is first ne- 

 cessary that the special bacterium which causes 

 the disease should get into the body. There 

 are several channels through which bacteria can 

 thus find entrance ; these are through the 

 mouth, through the nose, through the skfn, and 

 occasionally through excretory ducts. Those 

 which come through the mouth come with the 



