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SCIENCE 



[N. S. Vol. LIV. No. 1383 



tacked. It is this increasing complexity of 

 affairs which supports from the scientific side 

 the dictum that the physician is to treat the 

 patient rather than the disease for there is no 

 longer a type disease to be recognized regu- 

 larly. 



This brings us back to the original subject 

 of etiology. A careful biological study of 

 the many parasitisms of man and the higher 

 animals brings out the fact that the highly 

 specialized parasites have no obstacles to their 

 activity, except one and that is immunity, 

 either acquired or natural. As a result all 

 host individuals pass through the disease early 

 in life provided opportunity for invasion is 

 given. The highly specialized diseases tend to 

 become, in endemic localities, children's dis- 

 eases. 



In the case of many other diseases certain 

 non-parasitic factors are necessary to start the 

 disease or to check it, as the case may be. 

 They are part of the mechanism of causation 

 and represent necessary conditions. These 

 necessary conditions may far outweigh the 

 living agents in etiological significance. The 

 relative importance of the living factors may 

 be so low that their place may be taken by 

 other living agents or even non-parasitic fac- 

 tors in the sequence leading to or continuing 

 existing disease processes. This is probably 

 true of certain diseases of intestinal origin. 

 Such diseases are frequently described as due 

 to different microbic agents in different locali- 

 ties because the endemic flora happens to be 

 different. 



The relation between the factors of para- 

 sitism on the one hand and those of heredity, 

 environment and the like on the other may be 

 briefly summarized as follows: 



In the saprophytic or predatory type repre- 

 senting the so-called septic infections the other 

 parasitic and non-parasitic factors are of 

 great, even predominating importance in the 

 production of disease. 



In the highly parasitic type they are of 

 little, if any, importance unless it be heredi- 

 tary characters brought out by the selective 

 action of the parasites themselves upon the 

 host species through many generations. Many 



gradations exist between these extremes and 

 the relation of parasitic to extra-parasitic fac- 

 tors is different for the different grades. 



Moving parallel with the degree of adapta- 

 tion and specialization and the development 

 of more nearly perfect cycles by the parasites 

 the mortality drops and the morbidity at first 

 spreads and finally tends to decline in certain 

 types of parasitism, provided always that other 

 types of parasitism do not accidentally enter 

 to modify and complicate the normal course. 



In tracing the various living agents through 

 the body of the host we find that we do not 

 know the details of any parasitism to our 

 satisfaction. These details, of course, include 

 also the non-parasitic factors or conditions es- 

 sentially favoring or hindering the parasite 

 in its sojourn in and its journey through the 

 host tissues. It is perhaps needless to refer 

 at length to the conditions which control the 

 acquisition of such knowledge. The hope of 

 finding some preventive or cure dictates the 

 course of many workers. When something ap- 

 proaching this has been found the etiological 

 significance of all the other factors bearing 

 on the disease falls below the horizon for the 

 time being. The importance of continuing the 

 study of the disease persists however even for 

 practical reasons, for the remedy or pre- 

 ventive may not prove to have the success an- 

 ticipated. To induce men to fill the gaps of 

 our knowledge seems quite as important as the 

 pioneering for entirely new vistas or outlooks. 

 Great discoveries are, as a rule, half-truths 

 that must be brought into line by patient after- 

 research. The filling of gaps may be neces- 

 sary to stage the next great discovery. 



There is beyond the mere knowledge that 

 gaps exist the difficulties of the problems in- 

 volved to be considered. Are we prepared to 

 solve them directly or must we rely on indirect 

 approaches and on the solution of analogous 

 problems to satisfy our etiological sense? 

 Added to these difiiculties inherent in some 

 phases of the parasitic cycle, notably that 

 phase which takes place within the host, is 

 the fact that the various disease agents attack- 

 ing the same species, man for instance, are so 

 different from one another that we may safely 



