MECHANISM OF INFECTION AND IMMUNITY 231 



as do those which suffer extracellular digestion. In the latter instance, 

 the cellular poison of the bacteria is set free and in its death it becomes 

 most dangerous to its host. 



By far the larger number of bacteria which infect man do not 

 elaborate soluble toxins and for these we can have no antitoxin. Of 

 the other pathogenic bacteria there are many which, in first infections 

 at least, are not to any large extent devoured by phagocytes. The 

 members of this large class, which cannot be met with antitoxins or 

 disposed of by stimulated phagocytosis, must be dealt with by bacteri- 

 cidal and bacteriolytic enzymes. The potent poison which they con- 

 tain is set free and exerts its deleterious effect, which is determined 

 by the rapidity with which the bacterial cells are disrupted. It must 

 be evident that the development of powerful bacteriolytic enzymes at 

 a time when the body is filled with bacteria would be most disastrous. 

 The faster the invaders are destroyed, the more danger is there to 

 the host. This is well illustrated in typhoid fever, in which the bacillus 

 produces no soluble toxin and consequently there can be no antitoxin 

 developed and in which there is no increase in the phagocytes. The 

 greatest misfortune that happens in the progress of typhoid fever is 

 the rapid development of a powerful bacteriolytic enzyme and the 

 speedy destruction of the invading bacteria in large numbers. This is 

 true of plague and typhus, as well as of typhoid fever. It does not 

 apply to diseases due to soluble toxins, such as diphtheria and tetanus, 

 and probably not to those combated exclusively by phagocytes, if there 

 be such. 



The assertion has been made that the infectious diseases have bene- 

 fited the race by the destruction of the unfit. This idea I have com- 

 bated most vigorously since our study of typhoid fever in the army 

 in 1898. My colleagues and I found that out of 9,481 soldiers who 

 had previously been on the sick report and could not be regarded as 

 possessing standard health, 648, or 6.8 per cent., contracted typhoid 

 fever; whereas, out of 46,383 men who had no preceding illness, 7,197, 

 or 15.3 per cent., developed typhoid fever. More than 90 per cent, of 

 the men who developed typhoid had no preceding intestinal disorder. 



Under ordinary conditions the strong, busy man, especially the 

 one whose activities demand wide excursions from his home, is more 

 likely to become infected than the one whose sphere of action is more 



