CHAPTER XVII 



BACTERIA IN AIR, SOIL, WATER, AND FOOD 



The presence of bacteria in air, in soil, in water, 

 and in foods is constant, and under favorable con- 

 ditions for their growth and multipUcation may 

 become a serious menace to health. 



In air bacteria may always be found upon the 

 floating dust, the largest number occurring in 

 crowded dwellings and in the air nearest to the 

 surface of the earth in cities. In the higher altitudes, 

 as in the mountains, the number ofjjacte ria de- 

 creasesjKith the altitude. 



It is not only when the bacteria in the air belong to 

 the disease-producing class that they constitute a 

 source of danger, for the action of the bacteria con- 

 cerned in the process of decomposition and putre- 

 faction may be particularly dangerous in foods, and 

 must be especially considered in the care of foods. 



The theory of " air-borne " infection being 

 much disputed, has largely been supplanted by the 



172 



