THE ATTENUATED VIRUS, OR VACCINATION. 227 



were morbid entities. A virus was a unity. This 

 opinion has still its declared upholders. According to 

 Pasteur a virus has different degrees of virulence ; it 

 can pass from the weakest virulence to the maximum. 

 Modifying, at will, the virus of fowl cholera, Pasteur 

 inoculates some hens, for instance, with a virus too 

 attenuated to protect from death, but which neverthe- 

 less is effectual in securing them against a virus 

 stronger than itself. The second virus will preserve 

 them from the attacks of a third virus, and thus 

 passing from virus to virus they end by being gua- 

 ranteed against the most deadly virulences. The whole 

 question of vaccination resolves itself into knowing 

 at what moment a certain degree of virus attenuation 

 is a guarantee of protection against the mortal virus. 



It seems that between small-pox and cow-pox facts 

 of a similar kind take place. It is probable that 

 vaccination rarely gives perfect security against the 

 infection of a very malignant small-pox; moreover, 

 during epidemics of small-pox many persons who have 

 been previously vaccinated are attacked, and some 

 even die of the disease. 



As regards the practice of vaccinating fowls against 

 the cholera peculiar to them which, though it cer- 

 tainly is not of the same importance as human vac- 

 cination, is a scientifically capital fact we may hope 

 that whatever the differences of receptivity in different 

 races, or in different individuals of the same race, 



