CHAPTER XXIV 

 CONTAGIOUS DISEASES 



How are contagious diseases spread and how 

 may they be controlled? 



Bacteria and disease. Bacteria have a very important 

 bearing on health because of the part they play in causing 

 a number of common diseases. In some cases the bacterium 

 which causes the disease has been discovered under the 

 microscope and the cause of the disease is thus definitely 



FIG. 149, Types of disease bacteria : o, tuberculosis; b, diphtheria; c, typhoid 

 fever ; d, vibrio of cholera ; e, anthrax ; /, erysipelas ; g, pneumonia. 



known. It has been discovered for diphtheria, tuberculosis, 

 typhoid fever, pneumonia, cholera, lockjaw, whooping cough, 

 and blood poisoning. There are other diseases which are so 

 similar to bacterial diseases in their general behavior that it 

 is generally believed that they are caused by bacteria or 

 other microorganisms, although the bacterium has not been 

 discovered. Of this nature are scarlet fever, measles, mumps, 

 and smallpox. 



There is a great difference in people as to their suscepti- 

 bility to the action of these bacteria. Several people ex- 

 posed to the same disease may all take the bacteria into their 



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