PHENOMENA OF INFECTION. 73 



fasciata), malarial fever by a few varieties of another 

 (anopheles), etc. 



THE BODY IN RELATION TO INFECTIONS. 



We have just considered the exacting conditions 

 v^^hich an infectious agent must fulfill to provoke disease. 

 The fulfillment of these conditions, however, only 

 meets the requirements in so far as the micro-organism 

 is concerned, and therefore presents but one side of the 

 problem; the other side, the position that the body 

 occupies in relation to infections, is equally important, 

 and it is to this phase of the question that we now turn. 

 A few lines above, this statement was made, "it is a 

 matter of common observation that, of a number of 

 individuals exposed to the same infectious disease, not 

 all are attacked; and in those susceptible, the disease 

 presents extraordinary variations as regards its mildness 

 or severity. If the microbe were the sole factor con- 

 cerned, all exposed persons would be attacked, and all 

 taking the disease would suffer to the same degree; a 

 thing which everyone knows never happens. Moreover, 

 a person may resist the action of a pathogenic agent at 

 one time, and fall victim to it at another." The ques- 

 tion that naturally presents itself is, what brings about 

 such radical differences in the vulnerability of individ- 

 uals to infectious agents ? That it is not always conse- 

 quent upon modifications in the agents, we know from 

 observations upon patients, and experiments upon 



