BALANCED PATHOGENISM; EPIDEMIOLOGY 109 



Theobald Smith 1 has mentioned the diphtheria bacillus as an 

 organism which possibly exhibits a tendency toward a parasitic exis- 

 tence. The toxin of the diphtheria bacillus is not a poison specific 

 for man; many animals, as the horse and guinea-pig, are very sus- 

 ceptible to it. Yet the diphtheria bacillus is almost obligately a human 

 pathogen. The ever-increasing occurrence of avirulent, non-toxin 

 producing strains which are otherwise perfectly typical, and the 

 frequent occurrence of individuals whose serum contains small 

 amounts of natural antitoxin might be interpreted as an indication 

 that strains of this organism are becoming gradually accustomed to 

 a purely parasitic existence in the upper respiratory tract of man on 

 the one hand, and that man has acquired some specific resistance to 

 the microbe on the other hand. 



The tubercle bacillus (typus humanus) is an excellent example of 

 an exquisitely balanced pathogenic microorganism. Its metabolism is 

 not markedly different from that of the host and the typical disease 

 excited by it is focal, chronic, and slow-going. Years may elapse before 

 the host finally succumbs. The development of the organisms within 

 the tissues of the host does not appear to lead to the formation of 

 substances which arouse the latent offensive and defensive mechanism 

 of the host to acute antagonism. During this long period the tubercle 

 bacilli establish communication with the outside and, in a majority 

 of cases, countless myriads of bacilli escape from the host before 

 death removes him as a source of infection. Occasionally tubercle 

 bacilli become widely disseminated in the body, causing rapidly fatal, 

 generalized miliary tuberculosis. These organisms perish with their 

 host. 



It is well known that the virulence of bacteria, many kinds at least, 

 can be increased decidedly by passage from animal to animal by 

 providing an artificial portal of entry and of exit from animal to 

 animal. This is accomplished by injecting the organisms into a 

 first animal and reinjecting them, at brief intervals, into other animals. 

 In such instances there is a direct continuity of growth from animal 

 to animal, greater than is met with in naturally occurring infections. 

 It is worthy of note that bacteria of the "opportunist" type are, gen- 

 erally speaking, more successfully exalted in virulence under these 

 conditions than the progressively pathogenic forms. 



There is yet another feature of Pathogenism which is worthy of 

 note. From time to time almost any bacterial disease, for example, 



1 Theobald Smith, loc. cit. 



