﻿NO. 8 RECOVERY FROM INFECTION FLEXNER 3 



more severe, affect far fewer individuals. Beside this fact of vary- 

 ing individual susceptibility within a species is to be placed the 

 highly significant one of racial distinction. All sheep, except the 

 Algerian variety, are subject to anthrax infection; the Maltese goat 

 readily becomes a reservoir of the coccus which it excretes in the 

 milk, and which produces Malta or Mediterranean fever, while other 

 species of goat do not. Certain strains of wheat are not subject to 

 attack by the fungus that readily causes " rust " in other species, 

 and this quality of susceptibility or the reverse is transmitted as a 

 unit character. It is not known whether the phenomenon of suscep- 

 tibility in animals depends also upon so sharp an hereditary factor. 



Infection is an active process quite different from the mere pres- 

 ence upon a surface of the body of the parasite capable of causing 

 disease. We carry constantly upon our skin and mucous membranes 

 a whole host of potentially infectious bacteria which, for the most 

 part, do no harm whatever, and possibly, indeed, through preoccupa- 

 tion of the soil, ward off at times more definitely and highly injurious 

 parasites. And yet they are capable of malign activities. The whole 

 large and important group of pus-producing bacteria are our con- 

 stant guests, as are quite a number of other species, including the 

 germ that causes pneumonia. 



Thus we are obliged to conclude that the surfaces of the body 

 exposed to constant bacterial action possess a high degree of insus- 

 ceptibility to ordinary infections. This state, about which there 

 can be no question, is spoken of as local immunity. It depends, 

 moreover, on definite factors, some of a relatively coarse kind, that 

 operate only as long as they may be intact, and fail to operate 

 adequately when imperfect. The opening up of " portals of entry," 

 as they are called, of bacteria into the body follows upon the break- 

 ing down of this system of external defensive mechanisms. Thus 

 the mucous membrane of the nasal and buccal cavities, especially 

 about the masses of lymphatic tissue called the tonsils, which are 

 imperfectly provided with a complete epithelial investment, partic- 

 ularly in the young, constitutes the gateway into the body of the 

 infectious agents causing diphtheria, measles, scarlet fever, epidemic 

 meningitis, and probably poliomyelitis. These mucous membranes 

 are, obviously, so directly exposed to wide fluctuations of tempera- 

 ture, degree of moisture and dust-content of the air, as to become the 

 common seat of slight pathological conditions that depress their 

 protective powers. That an intact epithelial investment acts as a 

 barrier to certain of these infections is shown bv the fact that an 



