PREFACE 



I N this volume the writer has attempted to state the 

 main facts, so far as they have been established, 

 with reference to infection and immunity, with the 

 practical object in view of indicating the measures 

 necessary for the prevention of infectious diseases. 

 As the work is intended for non-medical readers, I 

 have avoided technical terms as far as practicable, and 

 when it has been necessary to use these have endeav- 

 oured to explain them. I have thought it best not 

 to enter upon a discussion of the theories of im- 

 munity, or to attempt to give an account of the re- 

 sults of recent investigations with reference to the 

 " antitoxins," " agglutinins," " precipitins," " bacterio- 

 lysins," etc. This line of investigation has, during 

 the past few years, been so prolific in surprising re- 

 sults, and so many new technical terms have become 

 necessary for the designation of the newly discovered 

 bodies of this class and for the understanding of 

 Ehrlich's " side-chain theory," which attempts to ex- 

 plain their mode of action, that this subject does not, 



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