HOW THE BODY GETS AND RESISTS DISEASES 427 



tact with objects that contain harmful germs, we are im- 

 pressed with at least 3 things: (1) how efficient our skin 

 is in keeping out the invading army of germs; (2) how 

 important it is to wash the skin, especially that of the 

 hands, so that we may remove the germs before they 

 get into our food; (3) how necessary it is to keep the nasal 

 passages and the mouth (including the teeth) clean, so 

 that germs may not develop there, and spread disease to 

 the other parts of the body. 



Why do not the germs of disease destroy man at once? 

 The answer is that there are two sides to the battle; the 

 body has its defenders, as well as its enemies. One of 

 these is the army of white corpuscles, which destroys 

 many of the disease germs that get into the blood. We 

 have already seen (cf. 378) that the white corpuscles are 

 like amebas, and that they are not only in the blood, but 

 get through the capillaries into the lymph. If a wound is 

 made in the skin or mucous membrane, the white cor- 

 puscles do their best to destroy the germs that enter the 

 wound. The body has not only the white corpuscles to 

 defend it, but also produces certain substances that act 

 as germicides ("germ slayers"). A great part of the 

 reason why we need to keep the body rested, well-fed, and 

 strong is that it may have an abundance of vigorous white 

 corpuscles, and that it may produce the right germicides 

 for the germs that attack it. Yet, in spite of everything 

 that the body can do, some dangerous bacteria may get 

 past the body's lines of fortifications; we then have a 

 disease. 



The effect which disease germs have upon the body is 

 due not so much to the germs themselves as to the sub- 



