98 



ELEMENTS OF GENERAL SCIENCE 



FIG. 49. A crowded city block 

 The dark areas represent buildings. These buildings 



The air is never free 

 from them, although 

 they are more plenti- 

 ful at some times and 

 in some places than in 

 others. Water is also 

 a prominent agency 

 in the distribution of 

 bacteria and will be 

 discussed in connec- 

 tion with our con- 

 sideration of water 

 supply. 



Certain kinds of 

 disease-producing bac- 

 teria are most com- 

 monly communicated 

 more or less directly 

 from person to person 

 (fig. 49). Insects are 

 agents of great im- 

 portance, and investi- 

 gations of the past 

 two decades have 

 given most interest- 

 ing evidence regarding 

 the ways in which 

 some of the most de- 

 structive diseases are 

 dependent upon them 

 for distribution. The 

 mosquito, flea, and 

 cattle tick are espe- 



house 1600 persons, and in such crowded quarters c i a ll y bad offenders 



it is difficult to prevent the spread of disease. After 

 Report of the Department of Health of Chicago 



this respect. 



