THE PREVENTION OF INFECTIOUS DISEASE. 427 



another. Virulent germs may persist longest 

 in the slime at the bottom of lakes and rivers 

 because in such places abundant food ma- 

 terial exists together with restriction of the 

 air supply. 



The competition with the bacteria of putre- 

 faction to which disease germs are subjected acts 

 in such a way that even in cesspools and dung- 

 heaps disease germs perish as a rule, so that 

 practically the disinfection of such localities is 

 to say the least needless. It only exception- 

 ally happens that the emptying of a cesspool is 

 followed by an outbreak of typhoid fever. Such 

 a proceeding as the application of milk of lime 

 to manure heaps affects sensible people merely 

 by exciting their sense of the ridiculous. 



Works of sanitation are effectual also in a 

 more indirect way. The human being who 

 has become immune through their agency may 

 himself have such an influence upon the in- 

 vading parasite that the virulence of the latter 

 becomes diminished. The virulence of disease 

 producing bacteria maintains its force or may 

 even be increased in susceptible animals, but 

 generally decreases and may disappear in the 

 process of adaptation to resistant or immune 

 animals, a fact that was first established by 

 Pasteur (cf. p. 175). 



Works of sanitation act upon saprophytes 



