CHAPTER V 



DISEASES PRESUMABLY CAUSED BY ULTRA- 

 MICROSCOPIC ORGANISMS 



Rabies.— This is a disease which attacks the lower 

 animals and man ; in the latter the disease is called 

 " hydrophobia." 



Causation.— The actual cause is at present not 

 known, as the causative germ belongs to the class of 

 ultra-microscopic organisms. In the lower animals 

 the disease is most commonly seen among dogs, 

 although horses, cattle, and sheep are quite commonly 

 affected. 



Although nothing is known of the poisonous 

 material which gives rise to this malady, yet ex- 

 periments show that it resides in the nervous system 

 of infected animals, and that it can be modified in 

 various ways. Thus light, air, and desiccation rapidly 

 destroy the virulence of rabic matter. Heat also has 

 the same effect. Exaltation of virulence may be 

 effected by passing the virus through a succession of 

 rabbits. After passage through a large number of 

 these animals the incubation period is gradually 

 shortened from about three weeks to a constant 



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