VIRULENCE AND ATTENUATION 307 



capable, at this time, of killing sheep; attenuated a little 

 more it ceases to be fatal to sheep, but still kills rabbits 

 and guinea-pigs. When it no longer kills adult guinea- 

 pigs, it still kills young guinea-pigs or young mice. This 

 is also true for other microbes. 



Virulence appears to us, therefore, to be an intrinsic 

 quality of which the microbe will be divested more and 

 more until it becomes harmless. But here is a fact 

 which proves that things are not as simple as they seem. 

 If virulence were only this, the different methods of 

 attenuation would destroy it in the same fashion, and 

 the order in which the different species of animals are 

 attacked would be always the same. But experiment 

 shows that this order varies according to the method of 

 attenuation. The anthrax bacteridium attenuated, for 

 example, with bichromate of potash, in the experiments 

 of MM. Chamberland and Roux, as we shall see at once, 

 may still kill the sheep or at least make them very ill, 

 leaving them in the latter case vaccinated, while it 

 produces no effect whatever on rabbits or guinea-pigs, 

 and does not even vaccinate them. It is exactly the 

 reverse of the behavior of the anthrax bacteridium 

 attenuated by growth at 42 to 43C., which kills guinea- 

 pigs and rabbits at a stage when it is harmless for the 

 sheep, and does not even vaccinate them. We obtain 

 the same results with spores of the anthrax bacteridium 

 attenuated by the action of a temperature of 35C., in a 

 liquid containing 2 per cent sulphuric acid. 



Thus virulence is not, as we might suppose, an absolute 

 quality, diminishing little by little after the fashion 

 of reserve food; it is a relative quality, in the estimation 

 of which not only conditions pertaining especially to the 

 microbe must be taken into account, but also those 

 pertaining to the nature, age, and as we shall soon see, the 

 individuality of the animal on which it is studied. 



