HOW PLANTS BENEFIT AND HARM MANKIND 179 



teria are those which cause disease. This they do by becoming 

 parasites in the human body. Millions upon miUions of bacteria 

 exist in the human body at all times — in the mouth, on the teeth, 

 and especially in the lower part of the food tube. Some in the food 

 tube are believed to be useful, others harmless; still others cause 

 decay of the teeth, while a few kinds, if present there, may cause 

 disease. 



It is known that bacteria, like any other living things, feed and 

 give off organic waste. This waste, called a toxin, is poison to the 

 hosts on which the bacteria live, and it is usually the production of 

 this toxin that causes the symptoms of disease. Some forms, 

 however, break down tissues and plug up the small blood vessels, 

 thus causing disease. 



Diseases caused by Bacteria. — It is estimated that bacteria 

 cause annually over 50 per cent of the deaths of the himrian race. 

 As we will later see, a 

 very large proportion of 

 these diseases might be 

 prevented if people were 

 educated sufficiently to 

 take the proper precau- 

 tions to prevent their 

 spread. These precau- 

 tions might save the lives 

 of some 3,000,000 of peo- 

 ple yearly in Europe and 

 America. Tuberculosis, 

 typhoid fever, diphtheria, 

 pneumonia, blood poison- 

 ing, syphilis, and a score 

 of other germ diseases 

 ought not to exist. A 

 good deal more than half 

 of the present misery of 

 this world might be prevented and this earth made cleaner and better 

 by the cooperation of the young people now growing up to be our 

 future home-makers. 



Germs or contagious diseases either enter the body by way 



A single cell scraped from the roof of the mouth and 

 highly magnified. The little dots are living bac- 

 teria, most of them comparatively harmless. 



