December 16, 1904.] 



SCIENCE. 



829 



wider and wider diiSusion of these para- 

 sites, or potential disease producers. The 

 few still highly mortal plagues would event- 

 ually settle down to sporadic infections 

 or else disappear wholly because of adverse 

 conditions to which they can not adapt 

 themselves. 



In this mutual adaptation of microor- 

 ganism to host there is, however, nothing to 

 hinder a rise in virulence in place of the 

 gradual decline if proper conditions exist. 

 In fact, it is not very difficult to furnish 

 adequate explanations for the recrudes- 

 cence and activities of many diseases to- 

 day, though the natural tendencies are 

 toward a decline in virulence. In the more 

 or less rapid changes in our environment 

 due to industrial and social movements the 

 natural equilibrium between host and para- 

 site established for a given climate, locality 

 and race or nationality is often seriously 

 disturbed and epidemics of hitherto spora- 

 dic diseases result. Typhoid fever will 

 serve as one illustration of my thesis. It is 

 ordinarily a sporadic infection, passing 

 from the sick to the well by direct contact. 

 Our knowledge that the infection of this 

 and other diseases is contained in the dis- 

 charges of the sick and a growing sense of 

 cleanliness led years ago to the large sys- 

 tems of sewerage, which have made a 

 crowded city life possible. But the re- 

 moval of sewage from our immediate sur- 

 roundings was the beginning of new 

 trouble. The sewage was led into water 

 courses from which drinking water came. 

 Hence the great epidemics in place of spo- 

 radic disease. The direct transmission of 

 the parasite on a small scale was largely 

 checked, but the indirect transmission 

 greatly favored. The dweller in cities with 

 unprotected water-supply is still further 

 endangered by the fact that the typhoid 

 bacilli returned in the water may repre- 

 sent more virulent varieties than those 

 handed down by his ancestors in rural com- 



munities. The motley population brought 

 together by migrations from all parts of 

 the globe bring the various races of bacilli 

 with them to be redistributed on a large 

 scale. 



Conditions may even create diseases arti- 

 ficially. Thus in childbirth, the physician 

 through want of cleanliness may in his ex- 

 amination actually inoculate a wounded 

 surface with streptococci or other septic 

 bacteria. In a hospital badly managed, 

 such germs may be made to pass artificially 

 through a series of individuals and their 

 virulence raised. In nature this could not 

 take place, because there would be no phy- 

 sician. Hence the transfer would not take 

 place. The history of maternity hospitals 

 before the period of asepsis in surgery is a 

 sufficient proof for the theory advanced. 

 Hospital erysipelas and hospital gangrene 

 were diseases artificially bred. With the 

 introduction of the principle of asepsis in 

 medicine and surgery the artificially cre- 

 ated diseases were destroyed, -because the 

 transportation facilities of the bacteria 

 were cut off. 



These illustrations indicate that so-called 

 natural law does not stand in the way of 

 our having highly virulent types of dis- 

 ease if we are ignorant enough to cultivate 

 them. The microorganism is sufficiently 

 plastic to shape itself for an upward as 

 well as a downward movement. Among 

 the most formidable of the obstacles toward 

 a steady decline of mortality is the contin- 

 ual movement of individuals and masses 

 from one part of the world to another, 

 whereby the partly adapted parasites be- 

 come planted as it were into new soil and 

 the original equilibrium destroyed. These 

 various races of disease germs become 

 widely disseminated by so-called germ car- 

 riers, and epidemics here and there light 

 up their unseen paths. Fortunately for 

 us, the conditions under which these micro- 

 organisms may establish themselves are in 



