2/2 BACTERIOLOGY. 



tions increase the difficulty of research, already 

 arduous, but the fundamental facts that have 

 been discussed are not thereby affected or in 

 any way altered. The body of a living organ- 

 ism offers relatively constant conditions, and 

 that explains why, in spite of the complexity 

 due to an alternation of generations in several 

 hosts, the parasites can remain relatively the 

 same. 



Infection and Contagion. 



I cannot conclude my examination into the 

 causes of infectious disease without referring 

 to still another feature of the external condi- 

 tions which may be very important practically. 

 Granting the existence of a given disposition 

 toward disease, disease germs can evidently 

 afford opportunity for the manifestation of 

 this disposition only when they come in con- 

 tact with it. That is the broad meaning of 

 the word infection. From this point of view, 

 accordingly, an unsuitable condition of the 

 general surroundings of life, such as air, 

 water, soil, and kind of nourishment, may be 

 of importance by virtue of being the means 

 by which the disease germ is first introduced 

 into the body. The quite various channels of the 

 mouth, the lungs, and the skin are available 

 for entrance. The organs affected in an in- 



