86 Z YMOTIC DISEASES. 



all these precautions, the living germs might enter our 

 bodies, grow and multiply there to our detriment, and 

 perhaps destroy us ; while a friend who took no such 

 pains to protect, himself, and may have been for hours 

 exposed to the infected air, might escape altogether, 

 and enjoy perfect health though surrounded by con- 

 tagion on every side ! 



We shall have to consider in another part of this 

 work what circumstances probably increase our lia- 

 bility to attack, and by what means we may improve 

 our chance of escape. 



The diseases in question are known asZymotic(Zi>iir), 

 a ferment, leaven), but under this head have been also 

 included affections due to malaria, maladies resulting 

 from changes in the food, and diseases depending 

 upon the introduction into the body of living parasitic 

 animal or vegetable organisms. This classification 

 is not altogether satisfactory, because diseases induced 

 by the introduction into the organism of bodies so 

 essentially distinct from one another, as for instance 

 the poison of small-pox, and the tape worm, should 

 scarcely be included in one class. At the same time, 

 when the subject comes to be carefully considered, it 

 must be admitted that there are many difficulties, while 

 the classification in question possesses many practical 

 advantages, and upon the whole has been found to work 

 well. If the conclusions arrived at in this work are 

 accepted, it will not be difficult to separate the fevers 

 and allied diseases from others which are at present 



