CHAPTER XX 

 DISEASES DUE TO HARMFUL ORGANISMS 



Some Kinds of Organisms Produce Poisons in the 

 natural course of their metabolic activities. Among the 

 minute organisms that lead parasitic lives on or within 

 the bodies of larger organisms are numerous varieties of 

 which this is true. These are injurious to their hosts. 

 Others seem not to affect their hosts one way or another. 

 The case presented by organisms parasitic on and pro- 

 ducing disease in plants forms the subject matter of much 

 of the last chapter. Here the discussion will be restricted 

 to injury produced in animals by microorganisms. 



The Ways in Which Microorganisms Cause Harm 

 IN Animals are principally three. They may grow and 

 multiply in food, generating poisons and so making it 

 dangerous. They may cause excessive putrefaction of 

 the intestinal contents, with resulting ill effects to the 

 body. Or they may grow on or within the body itself, 

 poisoning it directly. 



Food Poisoning. — From time immemorial individuals, 

 and sometimes whole communities, have been stricken 

 with disease as the result of eating spoiled food. Among 

 striking instances are the wide-spread poisonings which 

 have occurred in Europe from time to time, due to a 

 fungous growth on rye which contains a poison known as 

 ergot. When this rye was ground into flour and made 

 into bread, whole communities that consumed the bread 

 became ill and many died. 



Decomposition of protein foods by putrefactive bac- 

 teria in the presence of too little oxygen sometimes gives 



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