234 BACTERIOLOGY 



ever, nothing to hinder a rise in virulence in place of the gradual de- 

 cline, if proper conditions exist. In fact, it is not very difficult to fur- 

 nish adequate explanations for the recrudescence 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 environ- 

 ment due to industrial and social movements the natural equilibrium 

 between host and parasite established for a given climate, locality, 

 and race or nationality is often seriously disturbed and epidemics of 

 hitherto sporadic diseases result. Typhoid fever will serve as one illus- 

 tration 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 discharges of 

 the sick, and a growing sense of cleanliness, led years ago to the large 

 systems of sewerage, which have made a crowded city life possible. 

 But the removal of sewage from our immediate surroundings 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 sporadic 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 represent more virulent varieties than those handed 

 down by his ancestors in rural communities. 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 artificially. Thus in child- 

 birth, the physician through want of cleanliness may in his examina- 

 tion 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 vir- 

 ulence raised. In nature this could not take place, because there would 

 be no physician. Hence the transfer would not take place. The his- 

 tory 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 introduc- 

 tion of the principle of asepsis in medicine and surgery, the artificially 

 created 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 disease if we 

 are ignorant enough to cultivate them. The micro-organism is suffi- 

 ciently 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 continual movement of individuals 

 and masses from one part of the world to another, whereby the partly 



