356 BACTERIOLOGY. 



being in contact with others having virulent germs on 

 their persons or clothing. In such cases the bacilli 

 may sometimes live and develop for days or weeks in 

 the throat without causing any lesion. When we con- 

 sider that it is only the severe types of diphtheria that 

 remain isolated during their actual illness, the wonder 

 is not that so many, but that so few, persons contract 

 the disease. It indicates that very frequently virulent 

 bacilli are received into the mouth, and then either find 

 no conditions there suitable for their growth or are 

 swept away by food or drink before they could effect 

 a lodgement. 



Susceptibility to and Immunity against Diphtheria. 

 An individual susceptibility, both general and local, to 

 diphtheria, as in all infectious diseases, is necessary to 

 contract this disease. Moreover, the diphtheria poison 

 does not produce the same effect on the mucous mem- 

 branes of all persons. Age has long been recognized 

 to be an important factor in diphtheria. Children 

 within the first six months of life. are but little sus- 

 ceptible, the greatest degree of susceptibility being 

 between the third and the tenth year, while adults 

 are almost immune. An inherited susceptibility or 

 "family predisposition" to the disease has also been 

 observed. 



Long before the discovery of the Klebs-Loffler 

 bacillus it was a well known fact that two attacks of 

 diphtheria seldom occurred in the same individual within 

 short periods of time, and none of us would fear to 

 leave a convalescent case in the same room with one still 

 suffering from the disease. To what this natural suscep- 

 tibility or immunity is due is still only partially under- 

 stood; when we remember, however, that simply a slight 



