24 THE NATURE OF INFECTION t 



is a good deal of evidence to show that local conditions are of the 

 greatest importance in determining the occurence or non-occur- 

 rence of infection. It is thus well known that infection with the 

 cholera vibrio, the typhoid or the dysentery bacillus can only occur 

 from the digestive tract, while the gonococcus shows a marked pre- 

 dilection for the genital tract and the conjunctiva, and the meningo- 

 coccus for the upper respiratory tract. The staphylococcus and 

 streptococcus, on the other hand, as well as the plague bacillus may 

 infect from almost any point,, and the same probably is true of the 

 pneumococcus, although its special affinity is directed to the respira- 

 tory tract. It might be argued, of course, that the organisms in 

 question do not meet with more favorable conditions for infection 

 at the points where this usually occurs than would be the case 

 elsewhere, and that they infect from these points largely because 

 they are the only regions which are usually open to invasion. There 

 is no good evidence, however, to support such a claim, while a num- 

 ber of data go to show that there are unquestionably definite dis- 

 tricts which are more prone to become points of infection with specific 

 organisms than others, because of purely local conditions. The 

 gonococcus and diphtheria bacillus are thus incapable of producing 

 an infection through the skin, even when this has been previously 

 wounded at the point of contact with the organisms in question. 

 The cholera vibrio can infect only from the intestinal mucosa, 

 but not from the mouth, the esophagus, the stomach, or the genital 

 tract. In the stomach, indeed, an active multiplication of the 

 organism in question cannot occur, as the cholera vibrio is rapidly 

 destroyed by the hydrochloric acid of the gastric juice. 



While local conditions are thus unquestionably of moment in 

 determining a liability to infection and primary invasion on the part 

 of an organism, we still have no explanation why pathogenic organ- 

 isms may exist at these points without consequent infection. The 

 demonstration of certain preferences of localization for the growth 

 of an organism is, however, in itself an important point to establish, 

 for unless local conditions were such that the invader could at least 

 maintain itself, subsequent infection would of course be rendered 

 difficult. 



Infection from the normal stomach, where hydrochloric acid is 

 being produced during many hours of the day, would a priori seem 

 to be a difficult matter. In consequence of the active motility of 



