Bacteria as Man's Invisible Foes 61 



harmless bacteria, some being little balls, 

 some rods, and some spirals. Like other 

 bacteria, they grow at the expense of the 

 materials with which, under favorable con- 

 ditions, they come in contact, and like them 

 they produce new chemical compounds as the 

 result of their life processes. When they get 

 into the human body, the different species 

 grow in different ways, and produce different 

 kinds of chemical compounds, and this growth 

 or the poisonous substances which are formed 

 induce disease. 



Bacteria which can grow in the body and 

 do serious harm there are called pathogenic 

 or disease-inducing bacteria. The poisonous 

 chemical compounds which they set free as 

 they grow, are called toxines. 



Now, before we try to comprehend how 

 disease can be induced by bacteria, we ought 

 to understand what disease is. 



We have seen in the first chapter that the 

 human body is made up of several commu- 

 nities of cells, each community having ac- 

 quired the power of doing some special thing 

 for the good of the body as a whole, and that 



