PROBLEMS OF PATHOGENIC MICRO-ORGANISMS 233 



Flexner pointed this out, and denominated the invasion as a terminal 

 infection. I think that they may also be appropriately styled the 

 parasites of the diseased state. 



Among the second group we may place such diseases as Asiatic 

 cholera and the bubonic plague. The origin of the first is unknown. 

 The definite host of the second is probably the rat. 



Among the third class we have such groups of. diseases as tubercu- 

 losis, leprosy, syphilis, and glanders, on the one hand, and the erup- 

 tive diseases, on the other. The former are very chronic, protracted, 

 the latter acute, rapid in their course. In the eruptive diseases the 

 infection seems to depend solely upon the specific susceptibility of 

 the individual, and immunity is easily brought about by protective 

 inoculation. 



In tuberculosis and leprosy the mode of infection is evidently very 

 different from that of the group just mentioned. Prolonged exposure, 

 as in family life, seems necessary to successful infection, and even then 

 many exposed individuals escape. In tuberculosis, heredity plays 

 a very prominent part in the eyes of the physician, because the dis- 

 ease appears to propagate itself in families. This is probably due 

 to the necessity for more intimate association and repeated exposure 

 in order that the disease might appear. Here the disease is long 

 drawn out, the parasite may become in a sense individualized, and the 

 attack upon a new host may have to be made repeatedly. With these 

 highly parasitic forms the necessity for a frequent transfer to another 

 host is slight. In leprosy, the disease may last fifteen years to twenty 

 years, and then death ensues, usually as a result of the attack of the 

 secondary invaders. 



From the biologic standpoint which I have endeavored to present, 

 we may conceive of all highly pathogenic bacteria as incompletely 

 adapted parasites, or parasites which have escaped from their cus- 

 tomary environment into another in which they are struggling to 

 adapt themselves, and to establish some equilibrium between them- 

 selves and their host. The less complete the adaptation, the more 

 virulent the disease produced. The final outcome is a harmless par- 

 asitism, or some well-established disease of little or no fatality, unless 

 other parasites complicate the invasion. The logical inference to be 

 drawn from the theory of a slowly progressive parasitism would 

 be that in the long run mortality from infectious diseases would be 

 greatly reduced through the operation of natural causes. But morbid- 

 ity would not be diminished, possibly greatly increased, by the wider 

 and wider diffusion of these parasites, or potential disease producers. 

 The few still highly mortal plagues would eventually settle down 

 to sporadic infections, or else disappear wholly because of adverse 

 conditions to which they cannot adapt themselves. 



In this mutual adaptation of micro-organism to host there is, how- 



