DISEASE AND SANITATION 



133 



the reason for sealing preserves and canned goods. Why 

 did you put the plug of cotton in the test tube of milk 

 which you pasteurized? 



If we leave food upon our teeth the bacteria, which 

 are always present, live upon this food and produce acids 

 which hurt the covering of our teeth, called enamel. If 

 this action continues very long the enamel is destroyed 

 and then the bacteria of decay cause our teeth to decay. 

 For this reason we should wash our teeth at least twice 

 a day and always before going to bed. When the enamel 

 is destroyed the repairs which a dentist may make are 

 not lasting because the decay goes on around the filling 

 which he puts into the tooth. Thus the hole or cavity 

 becomes larger and larger until the tooth is destroyed. 



52. Disease and Sanitation. 



Some bacteria enter the body and, living upon the 

 .tissues, produce various diseases. All bacteria which 

 enter the body are not harmful and some are helpful. 

 Diseases which are given by one person to another by 

 means of bacteria are called contagious. Flies are car- 

 riers of disease and the common house fly, which is 

 shown magnified five times, is one of our worst enemies. 



The life history of the common house fly is very 



