CHAPTER XL 



BACTERIA AND DISEASE 



881. Dangers to life. Man's health is often assailed 

 by his inward appetites and desires. He is also exposed 

 to accidents and dangers from without. Formerly men 

 were in constant danger from wild animals, but now man 

 the noblest and most powerful living being is constantly 

 assailed and often conquered by the smallest and simplest 

 of living creatures. In the midst of his work he may be 

 stricken with a deadly disease because millions of tiny 

 creatures are poisoning the cells of his body. 



The microscope has revealed a world of tiny creatures of 

 an infinite variety of form and manner of life. Three of 

 the simplest forms are yeast, mold, and bacteria. All of 

 them are of importance to man. 



682. Yeast is a plant which consists of a single cell 

 scarcely larger than a red blood cell. The cells live upon 

 sugar, and begin a series of changes to return it to the air 

 and water for use 'as plant food. Were it not for this 

 provision, much plant food might encumber the earth in 

 the form of sugar and starch, and both man and animals 

 might starve. After the sugar upon which yeast cells feed 

 is used up, they remain in a dormant state, and some be- 

 come dried and pass off as dust. Some are always float- 

 ing about in the air ready to grow in anything containing 

 sugar. Yeast must have warmth and moisture for its 

 growth. So cold or dried fruit does not sour. Yeast is 



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