RETURN TO VIRULENCE 311 



becomes acclimated in the pigeon, makes it sick and 

 somnolent more quickly, kills it sooner, and the blood 

 of the last pigeon injected into the pig, manifests there 

 a virulence superior to that of the most infectious mate- 

 rial from a pig which has died of erysipelas, even if the 

 pig was naturally infected. Here we have then aug- 

 mentation of virulence for the pig by passing the or- 

 ganism through the pigeon. The maximum to which a 

 virus can attain by passage through a race is, therefore, 

 not always, the maximum for that race. 



There we have a case of augmentation, here is a case of 

 attenuation to which I wish especially to call attention. 

 Let us substitute the rabbit for the pigeon in this series 

 of experiments. The microbe becomes accustomed to 

 the rabbit; all the animals die, but if we inoculate pigs 

 with the blood of the last rabbits for comparison with 

 that taken from the first rabbits in the series, we find a 

 progressive diminution of virulence. Soon the blood 

 of rabbits, inoculated into pigs, no longer kills them; 

 it only makes them sick and leaves them vaccinated 

 against the fatal erysipelas. Entirely parallel facts 

 have been worked out with other microbes. They 

 furnish a method of attenuation of viruses by passages 

 through living species, and increase our means of action 

 in a field of studies the future of which will show its 

 astonishing fruitfulness. 1 



We have now come back, apparently, to a conclusion 

 already stated : Virulence is a state of perpetual becoming. 

 But how much we have developed this idea, and what 

 precision the new facts have given to it, and to the bond 

 of theory which has enabled us to unite them! In the 



1 This prediction has been more than fulfilled. Since this book was 

 written very wonderful advances have been made in bacterio-therapy, 

 the most striking of which have been the control of diphtheria and the 

 prevention of typhoid fever and of tetanus. Trs. 



