THE PREVENTION OF INFECTIOUS DISEASE. 419 



breathe. Dogs and rats are in part carrion feed- 

 ers ; rabbits, guinea-pigs, rats and mice live 

 crowded in narrow burrows or holes in which 

 such a stench prevails as would nauseate a hu- 

 man being. To conclude from negative experi- 

 ments that gaseous decomposition products 

 have no influence in diminishing man's resist- 

 ance to disease would be a gross laboratory fal- 

 lacy. The man who is accustomed to pure air 

 does in fact become ill in the bad air of the de- 

 fectively aerated, overcrowded and insufficiently 

 lighted dwelling of the proletarian. The in- 

 stinct for cleanliness is fostered by such hy- 

 gienic measures as can be directed to over- 

 coming these improper conditions and the pure 

 air itself affects favorably the respiratory or- 

 gans and occasions deep breaths to be taken. 

 The practical application of these facts together 

 with, above all, the introduction of pure water 

 has gradually changed into almost universal 

 cleanliness, the uncleanliness that was once 

 prevalent in continental Europe, and that had 

 crept in since the Thirty Years' War. A sup- 

 ply of good water free from all suspicion of 

 infection, removes any direct relations which 

 might otherwise exist between ourselves and 

 the infiltrated subsoil ; on the other hand a 

 bad well in the neighborhood might bring 

 us into direct contact with contaminated soil. 



