THE ATTENUATED VIRUS, OK 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, 



Q 2 



