MODE OF SPREAD OF INFECTIVE DISEASES. 749 



dividuals, while others equally exposed to the attack of 

 the infective agents are not affected, in other words are 

 immune. 



As to the causes and the meaning of this individual 

 predisposition and immunity we know as yet very 

 little, but the results of a large number of experiments 

 on animals give us some insight into the behaviour of 

 the infective agents in the body. Further investiga- 

 tions must, however, be made on these points before we 

 can come to any very definite conclusions. The factors Factors which 

 which influence the predisposition of the body lie partly flnence. 

 in the body itself, in the constitution of its cells, secre- 

 tions, &c., and come partly from without, these external 

 factors aiding the infective agents in that they render 

 the body abnormally susceptible to attack. 



In the first place it may happen that the access of Protective 

 certain infective agents to their specific points of invasion agSnft thT S 

 is rendered difficult by the natural protective arrange- S^^ctCTiato 

 ments of the body ; thus the gastric juice can, according certain points 

 to its degree of acidity, and according to its quantity, 

 injure those infective agents which must develop in the 

 intestine ; and this injurious action may be present to 

 a greater degree in one species of animal, or in one 

 individual than in other individuals, where the infective 

 agents more easily pass this protective arrangement. 

 Further, as regards those infective agents which have 

 their seat in the lungs, the more or less narrow and 

 convoluted entrance to the respiratory tract, the ciliated 

 epithelium, and the sensitive mucous membrane form a 

 protective arrangement of varying efficacy, and according 

 to the development of these arrangements the one species 

 or the one individual will have an advantage over the 

 other as regards the ease with which parasitic bacteria 

 can enter the lungs. 



Further differences in the constitution of the points Differences in 

 of invasion even of an apparently trivial character must the characters 



. ' a r -, of the Points 



exercise a very important influence. Thus according to of invasion 

 species and race, according to the age and state of the:Tlselves - 

 nutrition of the individual, &c., there are probably 

 differences in the skin, in the mucous membrane, and 



