176 The Story of the Bacteria 



the air we breathe, and the water we drink. 

 If any of these necessities of life contain 

 the living germs of these diseases, there 

 is a liability of the infection of healthy or 

 predisposed individuals. 



The liabilty to acquire these diseases is 

 always increased in direct proportion to the 

 crowding together of the sick and the well 

 under unsanitary conditions in large communi- 

 ties. This is not because filth and dirt are in 

 themselves infectious, but because pathogenic 

 bacteria are liable to become mingled with the 

 rest. In other words, there is a simply filthy 

 filth, and there is a pathogenic filth, and the 

 two are very apt to go together. 



No gas, however foul, no accumulation of 

 dirt, no degree of malnutrition or misery or 

 overcrowding can induce an infectious disease. 

 It is always and everywhere some particular 

 form of disease-producing germ which causes 

 the trouble. The other influences bear largely 

 upon the chances of incurring the disease, and 

 often determine the severity of its course or 

 its fatal ending, but they alone cannot cause it. 



It is not the great and sweeping epidemics, 



