830 



SCIENCE. 



['N. S. Vol. XX. No. 520. 



many eases so complex that they can not 

 be realized. It is highly probable that 

 the bubonic plague can not get a foothold 

 or maintain itself among us, while Asiatic 

 cholera might have a better chance, through 

 our still greatly unsatisfactory water-sup- 

 plies. Many tropical diseases would fail 

 to take root in our climate. The mys- 

 terious rise and disappearance of leprosy 

 in the middle ages has astonished many 

 students of epidemiology. Possibly some 

 slight bias of the microorganism 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 go- 

 ing 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 sus- 

 ceptible to others. Of great interest is 

 the effect upon the human race of the as- 

 siduous care of those afflicted with certain 

 chronic diseases which is just now ex- 

 pressing itself in the establishment of 

 sanatoriums for the cure of the tubercu- 

 lous. 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 vac- 

 cination experiment to whose beneficent in- 

 fluence the century just past bears ample 

 testimony. The vaccinated individual is 

 either wholly immune or else the disease 

 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 improbable 

 that in the mildest cases it may not reach 

 that maturity necessary for the successful 

 infection of others. In view of the adapta- 

 bility of microorganisms 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 vaccina- 

 tion in infancy is not an adequate pro- 

 tection during life and at least one nation 

 —a nation which not only cultivates but 

 consistently utilizes science — prescribes two 

 vaccinations as necessary to complete pro- 

 tection. Whether in the days of Jenner 

 repeated vaccinations were deemed neces- 

 sary I have not been able to verify; but 

 we may assume without immediate fear 

 of experimental contradiction that a cen- 

 tury of incomplete protection may have 

 worked some changes in the smallpox or- 

 ganism. In any case, it is obvious that our 

 thesis implies in addition to the natural 

 decline of virulence also a gradual rise in 

 virulence whenever the resistance of sus- 

 ceptible individuals is raised on a very 

 large scale. Either the microorganism if 

 a true parasite will perish or else it will 

 augment its invasive powers to meet those 

 of its host. 



Another problem has been created for 

 the diphtheria bacillus by the extensive 

 use of diphtheria antitoxin. Will the 

 thorough protection of one group of human 

 beings lead to the decline or to the increase 

 in virulence of the diphtheria bacillus cir- 

 culating among the individuals of this 

 group? What effect will the transfer of 

 such bacilli to unprotected groups have? 

 These and similar queries may be answered 

 not many years hence, for a generation of 

 microbes represents a very short space of 

 time. 



It may not be out of place to call atten- 



