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PART VII. 



THE MODE OF SPREAD OF THE INFECTIVE DISEASES. 



The principal IN the following paragraphs we shall sketch shortly 

 modeofspread the mode of spread of the human infective diseases, hut 

 tfvefdiseases on ^ y in so fai> as ^ bears an intimate relation to the 

 biological facts mentioned in the preceding pages, to the 

 distribution of the pathogenic bacteria, and to the con- 

 clusions already drawn. And even within these limits 

 we shall by no means attempt to give a complete de- 

 scription in view of the large number of these diseases 

 which never completely coincide in their mode of spread, 

 of the number of the problems to be solved, of the 

 numerous isolated facts obtained by investigation and 

 observation, and of the manifold hypotheses and theories 

 which have been promulgated. This limitation will be 

 the more permissible, because in a former chapter on 

 " Cholera " a detailed description of the mode of spread 

 of at least one kind of infective agent has been given, 

 and to this we must refer the reader as an example 

 which may be extended on the fundamental lines to be 

 mentioned below. 



In considering the mode of spread of the infective 

 diseases we have in the first place to take into con- 

 sideration the sources of infection, the modes of trans- 

 port to man, and the points of invasion at which the 

 penetration of the infective agents into the healthy body 

 occurs. Then we have to pay special attention to the 

 individual predisposition and immunity, for these points 

 influence the mode of spread of many infective diseases 

 in a high degree. Finally, we must shortly consider 

 the meaning and the causes of the local and seasonal 

 variations in the epidemic distribution of the infective 

 diseases. 



