DUST AND ITS DANGERS. 53 



harmless species with which dust is usually 

 swarming. But few as they are they have an 

 extreme significance. If it were not for these 

 few species of disease-producing bacteria most 

 people could perhaps afford to be as indifferent 

 as they are to dust and its dangers. 



We have seen that the large numbers of 

 common bacteria which are omnipresent in the 

 air are growing all about us and get into the 

 dust in many ways which we not only cannot 

 control but do not very much care to control, 

 since in moderate numbers they are essentially 

 harmless, or at least do only such damage as 

 other inorganic dust particles may do. But 

 with the bacteria which cause disease the case 

 is entirely different. They do not flourish 

 apart from the bodies of men and animals. 

 They may remain alive for a good while out- 

 side of the body, and some of them may grow 

 a little under some few special conditions. 

 Some of them are frequently present in the 

 healthy human body. 



But after all when we seek for the active 

 breeding-places and sources of distribution of 

 the bacteria which frequently cause disease in 



