CHAPTER XLIII 

 INFECTIOUS DISEASES 



720. Resistance to infectious diseases. All infectious 

 and contagious diseases are caused by living germs (p. 385). 

 Our body always has some power of protecting itself against 

 disease germs, for the white blood cells and the blood plasma 

 attack them. If the body is injured in any way, the germs 

 may succeed in growing. Then the body produces great 

 numbers of new white blood cells, and also develops anti- 

 toxins against the poisons of the disease (p. 386). Thus, 

 as a disease develops, the body usually increases its powers 

 of resistance, and so most cases of infectious diseases end 

 in recovery. The white blood cells finally overcome the 

 disease germs, the liver, skin, and kidneys throw off the 

 poisons of the disease, and thus the disease ends (p. 248). 



Often the power of resisting and overcoming the germs 

 of a disease lasts for the rest of a lifetime. Thus a person 

 seldom has smallpox or scarlet fever or measles twice. In 

 some other diseases, as in diphtheria, the resistance lasts for 

 only a few weeks or months. If a person's body can pre- 

 vent the germs of a disease from growing, that person is 

 said to be immune to that disease. 



721. Immunity. A few persons are born with such a high resist- 

 ance to diseases that they escape measles, scarlet fever, whooping cough, 

 and other common diseases, although they may be exposed to them. 

 Other persons have such a low resistance to diseases that they readily 

 take diseases to which they may be exposed. Those who have adenoids 



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