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BACTERIA AND DISEASE 



What causes Disease ? — There are many causes of disease, 

 although there is usually only one primary cause in each instance. 

 A person may so abuse his body machine through lack of sleep or 

 exercise or proper food that soon his body will not function prop- 

 erly. He may become nervously out of sorts and imagine himself 

 ill, and succeed pretty well in making the delusion a reality. He 

 may poison his body with alcohol or nicotine and injure some of his 

 internal organs so that he never recovers his former efficiency. 

 He may meet with an accident and be crippled or otherwise in- 

 jured. Or he may be attacked by some of man's microscopic 



foes, bacteria or protozoa, and suffer 

 from an infectious disease. The last- 

 named cause is by far the most usual 

 and may be said to induce much more 

 than half of the common ills that cause 

 pain and sorrow. 



How we get Bacteria for Study. — 

 If any one should express doubt that 

 bacteria cause disease, it is easy to 

 prove that they do. But we must be- 

 gin by proving that there are such 

 things as bacteria. Since these tiny 

 plants, '' man's invisible friends and 

 foes," are to be found " anywhere but 

 not everywhere " in nature, it is easy 

 to obtain them with the proper means. 

 To obtain cultures of bacteria for study, it is first necessary to 

 find some material in which they will grow, then to kill all living 

 matter in this food material by heating it to the boiling point (212° 

 Fahrenheit) for half an hour or more (this is one method of steriliza- 

 tion), and finally to protect the culture medium, as this food is 

 called, from other living things that might feed upon it. 



Many bacteria thrive in a mixture of beef extract and gelatin 

 or agar-agar, a substance derived from seaweed. This mixture, 

 after sterilization, is poured into flat dishes with loose-fitting 

 covers which have been previously sterilized also. These Petri 

 dishes, so called after their inventor, are the traps in which we 

 collect and study bacteria. 



Steam sterilizer. 



