PROBLEMS OF PATHOGENIC MICRO-ORGANISMS 235 



adapted parasites become 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-carriers, and epi- 

 demics here and there light up their unseen paths. Fortunately for 

 us, the conditions under which these micro-organisms may establish 

 themselves are in many cases so complex that they cannot be real- 

 ized. It is highly probable that the bubonic plague cannot get a foot- 

 hold or maintain itself among us, while Asiatic cholera might have a 

 better chance, through our still greatly unsatisfactory water-supplies. 

 Many tropical diseases would fail to take root in our climate. The 

 mysterious rise and disappearance of leprosy in the Middle Ages has 

 astonished many students of epidemiology. Possibly some slight 

 bias of the micro-organism may have accomplished what seems almost 

 a miracle. Perhaps the right race or variety, once introduced, may 

 repeat the history of the Middle Ages in our day or in that of the 

 coming generation. 



Another obstacle to the amelioration of infectious diseases is the 

 rapid change going on in the habits of individuals and the ferment 

 in our conceptions of health and well-being, which are continually 

 upsetting any established equilibrium and making us more resistant 

 to some diseases, more susceptible to others. Of great interest is the 

 effect upon the human race of the assiduous care of those afflicted 

 with certain chronic diseases which is just now expressing itself in 

 the establishment of sanatoriums for the cure of the tuberculous. 

 If this movement should gain great headway, there may be a race of 

 immunes gradually developed who may be able to stand the untoward 

 conditions of indoor city life much better than the naturally robust 

 and physically superior who have no so-called hereditary taint. 



Of still greater interest is the vast vaccination experiment to whose 

 beneficent influence the century just past bears ample testimony. 

 The vaccinated individual is either wholly immune, or else the dis- 

 ease contracted after exposure is abortive, and the eruptive stage 

 does not come to full development or maturity. The excretion of the 

 infecting organism is thereby greatly interfered with, and it is not im- 

 probable that in the mildest cases it may not reach that maturity 

 necessary for the successful infection of others. In view of the adapt- 

 ability of micro-organisms in general, it is not beyond the range of 

 possibility that a variety of the smallpox organism may through a 

 chain of accidents arise as a result of successive passages through 

 partly protected individuals. To-day it seems fairly well established 

 that a single vaccination in infancy is not an adequate protection 

 during life, and at least one nation a nation which not only culti- 

 vates but consistently utilizes science prescribes two vaccinations 

 as necessary to complete protection. Whether in the days of Jenner 

 repeated vaccinations were deemed necessary I have not been able to 



