14 INFECTION AND RESISTANCE 



However, it is only very definite species of micro-organisms 

 which can cause disease at all when introduced into the body by 

 these paths. For, although the rubbing of plague bacilli into the 

 skin, or the inoculation of a cut surface with streptococcal or glan- 

 ders bacilli, will rapidly lead to progressive infection, similar inocu- 

 lation with the typhoid bacillus or the cholera spirillum would lead 

 to no such result. And, though the swallowing of pus cocci, pneu- 

 mococci, and a number of other micro-organisms would be entirely 

 without effect, similar ingestion of the typhoid and cholera organism 

 would usually result in typical infection. 



The path of introduction, therefore, is an important considera- 

 tion in determining whether or not a given micro-organism may give 

 rise to disease. It is necessary that the manner of gaining entrance 

 be suited to the cultural and other peculiarities of the germ in ques- 

 tion. In the case of cholera, for instance, the spirillum which causes 

 this disease is peculiarly susceptible to the deeper defences residing 

 in the body fluids and cells, and cutaneous infection by the small 

 numbers of bacteria likely to be introduced in this way would 

 promptly be checked by these agencies. In the intestinal mucosa, 

 however, the cholera spirillum finds conditions most favorable for 

 rapid multiplication, and the disease is caused by the inflammation 

 and destruction of the mucous and submucous tissues by the poison- 

 ous substances emanating from the large numbers of cholera spirilla 

 which die and are disintegrated, as well as by the absorption of these 

 poisons into the circulation. The bacteria themselves, however, never 

 gain a permanent foothold within the blood or other organs. In the 

 case of typhoid fever the conditions are somewhat similar, although 

 here, during the earlier weeks of the disease, we have an actual 

 penetration of the bacilli into the circulation. This, however, prob- 

 ably takes place only after intraintestinal proliferation has taken 

 place, which then, on' the injured mucosa, represents a dose out of 

 all proportion great when compared with the quantities that would 

 spontaneously come into contact with the external surface of the 

 body. 



This leads us to another important factor concerning the invad- 

 ing forces, in the determination of successful infection, namely, that 

 of the quantity introduced or the dosage. 



In order to cause infection, even when the bacteria are of the 

 variety known to produce disease or "pathogenic," and are brought 

 into contact with the body by a path suitable to their peculiar re- 

 quirements, the initial quantity introduced must be sufficiently large 

 to preclude complete annihilation by the first onslaught of the de- 

 fensive powers of the body. It is plain, therefore, that in the case 

 of bacteria weak in power to cause disease, given the subject of in- 

 fection and his defences as a constant, the quantities to be introduced 

 must be larger than in the case of micro-organisms of violent disease- 



