THE ATTENUATED VIRUS, OK VACCINATION. 223 



virulent virus that, for instance, which was just now 

 mentioned as capable of killing its hundred per cent, 

 of those inoculated with it, in twenty-four or forty- 

 eight hours these fowls would perhaps become rather 

 ill, but they would not die. The conclusion is simple ; 

 the disease can protect from itself. It has evidently 

 that characteristic of all virulent diseases, that it 

 cannot attack a second time. 



However curious it may be, this characteristic is 

 not a thing unknown in pathology. Formerly it was 

 the custom to inoculate with small-pox to preserve 

 from small-pox. Sheep are still inoculated to preserve 

 them from the rot ; to protect horned cattle from 

 peripneumonia they are inoculated with the virus of 

 the disease. Fowl cholera offers the same immunity ; 

 it is an additional scientific acquisition, but not a 

 novelty in principle. 



The great novelty which is the outcome of the 

 preceding facts, and which gives them a distinct 

 place in our knowledge of virulent diseases, is that we 

 have here to do with a disease of which the virulent 

 agent is a microscopic parasite, a living organism 

 cultivated outside of the animal body, and that the 

 attenuation of the virulence is in the power of the 

 experimenter. He creates it, he diminishes it, he 

 does what he wishes with it ; and all these variable 

 virulences he obtains from the maximum virulence 

 by manipulation in the laboratory. Looked at in 



