CHAPTER IX 

 DISTRIBUTION OF BACTERIA AND OTHER DISEASE GERMS 



98. Distribution of bacteria. There are many ways in which 

 bacteria may be distributed, but one of the most common of 

 these is by means of the air. It often happens that substances 

 upon which bacteria are growing become dry and powdered, as 

 in the street dust of which we have spoken. The bacteria are 

 then blown about with the remainder of the dust, to settle 

 wherever they may. The air is never free from them, though 

 they are more plentiful at some times than at others. Also, 

 there are some places where they are not abundant, as in the 

 air over deserts and over the ocean. Water is also a promi- 

 nent agency in the distribution of bacteria, and this will be 

 discussed in connection with our consideration of water supply. 



Certain kinds of disease-producing bacteria are most com- 

 monly communicated more or less directly from person to 

 person (fig. 47). Insects are agents of great importance in 

 many cases. The common house fly is an especially bad 

 offender in this respect. 



99. Bacteria upon carelessly handled bread. A special stu- 

 dent of bacteriology recently made a careful study of loaves 

 of "bread, to ascertain how many bacteria there are upon the 

 surfaces of the loaves. This student secured bread from vari- 

 ous parts of the city and from people who handled and sold it 

 in many ways. Unwrapped bread had more bacteria upon it 

 than wrapped bread. From 31 loaves collected from dirty 

 shops the average number of bacteria upon each loaf was 

 64,970. Of the unwrapped samples 39 per cent had an aver- 

 age of 14,000 bacteria upon each loaf, 39 per cent had an 

 average of 4000 bacteria upon each loaf, and the remaining 



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