THE RELATIONS OF BACTERIOLOGY 213 



will become a national scourge, or who looks to see the citizens of 

 London driven into the fields by the Black Death? It is of course 

 true that the continuance of this immunity can be secured only by 

 unremitting watchfulness, although so long as existing conditions of 

 civilized life are maintained, the recurrence of great epidemics may 

 be relatively remote. The pestilences that once stalked boldly through 

 the land slaying their ten thousands are now become as midnight 

 prowlers seeking to slip in at some unguarded door within which lie 

 the young and the ignorant. Already some once-dreaded maladies 

 have become so rare as to rank as medical curiosities, and their ulti- 

 mate annihilation seems assured. 



There are other diseases, however, that civilized life, or at least 

 modern life, appears to leave substantially unchecked, and some that 

 it even fosters. These may be considered as shining marks for the 

 modern hygienist. The scale between hygienic gain and loss is always 

 in unstable equilibrium. There is no such thing as consistent improve- 

 ment all along the line. As Amiel wrote in his journal, "In 1000 

 things we advance, in 999 we fall behind; this is progress." It is al- 

 most a biologic axiom that progress in one particular entails loss in 

 others. To maintain the efficiency of all parts of the complex of civili- 

 zation calls for eternal vigilance. It may be that while we are waxing 

 complacent over the fact that the opportunities for infection with cer- 

 tain parasites are diminishing, and that other parasites are gradually 

 losing what we vaguely denominate as their virulence, unforeseen and 

 greater evils are raising their heads. The increasing exemption from 

 certain diseases will itself lead to an increased prevalence of others as 

 diversely vulnerable age groups are formed. In general, it will occur 

 that the diseases peculiar to the advanced age groups will increase 

 as the diseases of childhood and youth succumb to hygienic measures. 

 A different age distribution of the population will bring in its train 

 new problems of preventive medicine, which must be successfully 

 solved if the issue is to be fairly met. 



There are not lacking instances of a dawning consciousness on the 

 part of mankind that the proper development of public hygiene 

 involves a far more comprehensive view of its relations than has 

 hitherto been taken. The study of tuberculosis is being approached 

 by methods of unexampled broadness. We are just beginning to re- 

 cognize the way in which the roots of this destructive malady are 

 well-nigh inextricably interwoven with the whole social fabric. Bac- 

 teriologic, architectural, and economic data are all levied upon for 

 contribution to our knowledge of what is universally recognized as 

 one of the most important of all human diseases. Here, as elsewhere, 

 the care and cure of the infected individual still looms large, but 

 beyond and above this is beginning to be placed the prevention of 

 infection, the drying-up of the stream at its source. That for this 



