CHAPTER IT 

 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 infection 

 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 absolute 

 natural immunity really exists among individuals of a species which 

 is known to be generally susceptible to infection with a given organ- 

 ism, 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 pneumococcus, 

 influenza bacillus, staphylococcus, streptococcus, and diphtheria 

 bacillus, infection even 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 

 assumption 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 conceivable 

 that local conditions may exist which would prevent the penetration 

 of a microorganism into the deeper tissues from certain surfaces, 

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



