104 THE STORY OF THE BACTERIA. 



the water we drink. If any of these necessi- 

 ties of life contain in them the living germs of 

 these diseases, there is a liability of the infec- 

 tion of healthy or predisposed individuals. 



The liability 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. 



In considering what ought to be and what 

 can be done to prevent the spread of the bac- 



