CHAPTER II. 

 THE NATURE OF INFECTION. 



IN studying the subject of infection, one of the first questions 

 which naturally suggests itself is: Why does infection not always 

 follow primary invasion? In some cases it might be argued that 

 the invasion at the time of observation was not primary, and that 

 the person in question may have acquired immunity to the micro- 

 organism under consideration at an earlier date, but that the infec- 

 tion at the time was so mild as to have escaped detection. In the 

 case of such organisms as the typhoid bacillus, the plague bacillus, 

 and the cholera bacillus, such an explanation might be warrantable 

 in some cases, but it is evidently an explanation for which proof 

 would be difficult, if not impossible to furnish; it would be a mere 

 assumption without any adequate basis. 



Then, again, it might be argued that infection does not occur 

 owing to the existence of a natural general immunity; but, as a matter 

 of fact, it is extremely doubtful on the one hand whether an abso- 

 lute natural immunity really exists among individuals of a species 

 which is known to be generally susceptible to infection with a given 

 organism, and, on the other hand, we find that some individuals 

 actually do become infected at a later date, showing that they were 

 in reality not immune. With such organisms, moreover, as the 

 pneumoccus, influenza bacillus, staphylococcus, and streptococcus 

 infection often does not give rise to an immunity that is deserving 

 of the name; on the contrary it leads to hypersensitiveness and not 

 to increased resistance. We can accordingly discard the assump- 

 tion that a general immunity is an important factor in discussing 

 the reasons why primary invasion does not always lead to actual 

 infection. 



Local Conditions and Infection. On the other hand it is inconceivable 

 that local conditions may exist which would prevent the penetra- 

 tion of a microorganism into the deeper tissues from certain surfaces, 

 while from others this would be possible. As a matter of fact there 



