248 LOUIS PASTEUR. 



fowls, its virulence can be restored by inoculating 

 small birds. Blackbirds, canaries, sparrows, all die, 

 if the virus has not been too much attenuated ; and 

 the effect is similar on young chicks. Thus by 

 several successive transitions from bird to bird a viru- 

 lence may be fostered capable of destroying full-grown 

 fowls. 



These facts suggested to Pasteur certain inductions 

 which may be well founded. Is not the attenuation of 

 the virus by the influence of the air one of the factors 

 in the extinction of great epidemics ? And may not 

 the reappearance of these scourges be accounted for 

 by the reinforcement of the virulence ? 



' The accounts which I have read,' Pasteur re- 

 marked some months ago, ' of the spontaneous appear- 

 ance of the plague in Benghazi in 1856 and in 1858 

 tend to prove that this outbreak could not be traced 

 to any original contagion. Let us suppose, guided by 

 the facts now known to us, that the plague, a malig- 

 nant disease belonging to certain countries, has germs 

 of long duration. In all these countries its attenuated 

 virus must exist, ready to resume its active form 

 whenever the conditions of climate, of famine, of 

 misery present themselves afresh. The condition of 

 long duration in the vitality of the germs of evil is 

 not even indispensable; for, if I may believe ilu 

 doctors who have visited these countries, in all places 

 subject to the plague, and in the intervals of the 



