Bacteria as Man's Invisible Foes 67 



into the body and proliferation there of 

 pathogenic micro-organisms is called an in- 

 fectious disease; the organisms, the infective 

 agents. These organisms are most frequently 

 bacteria, but other minute plants, such as 

 yeasts and animals called protozoa, are 

 sometimes to blame. 



Each infectious disease has its special 

 features by which the doctor knows it. 

 These features depend upon the species of the 

 invading germ, its way of growing, the poison 

 it sets free, the length of its life span. But 

 the body cells have their particular vulnera- 

 bilities to microbic invasion and poisons. 

 So that in one case it is the nervous system, 

 in another the lungs or the blood or the 

 digestive system which especially suffers. 

 Moreover, as one rose is redder than another, 

 or one aromatic plant more pungent than its 

 fellow, so in one case the germs which gain 

 access to the body may involve a more 

 potent poison than do those of the same 

 species but of another strain, and then the 

 disease may be more virulent in type. So 

 also an individual may at the time of in- 



