THE PREVENTION OF INFECTIOUS DISEASE. 405 



of their most worthy members, while the more 

 worthless individuals prove to be adapted ; in- 

 fectious diseases do not remove with certainty 

 merely the anti-social elements of the com- 

 munity, while leaving and bringing to matu- 

 rity only the socially useful. Hygiene has 

 accordingly undertaken the task of removing 

 these injurious influences. This reform may 

 be brought about either by making war upon 

 the disease germs, or by combating those con- 

 ditions of life which are of importance for the 

 disposition either of man or of the parasites, 

 or which have an influence through the fact 

 that they make possible the transmission of 

 the parasites to man. 



Measures of hygiene bearing directly on 

 the cause may be applied to different links of 

 the chain to prevent its closing, that is to 

 say, to prevent the outbreak of disease in 

 individuals or groups of individuals. In this 

 method of contest with disease many more 

 things prove useful practically than might be 

 theoretically supposed, and pure reason needs 

 to be supplemented by practical common 

 sense. Experience has certainly a word to 

 contribute. Since detailed considerations be- 

 long to the more restricted province of hygiene, 

 I shall simply make some brief statements, for 

 the sake of completeness, to illustrate the in- 



