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-Léffler 
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 aslight . 
