140 TEXT-BOOK OF BACTERIOLOGY. 



disease, small-pox, by inoculating persons, after a special prepara- 

 tion, with genuine small-pox virus, and that they were able, by 

 suitable measures, to attenuate the effects of the infection, and by 

 these means insure those so treated against the fliU force of a na- 

 tural attack of this dreaded malady. 



This process was successful, it is true, but it was nevertheless 

 dangerous and defective. In lieu of it came, as is well known to 

 all, toward the end of the last century, the practice of vaccination 

 with the so-called cow-pox virus, as introduced by Jenner. Whether 

 the cow-pox virus is identical with or nearly or distantly related 

 to the genuine small-pox virus is still an unsolved question. Suf- 

 fice it to say that the disease called cow-pox gives immunity from 

 the variola vera, and this fact has led to very important discov- 

 eries, which we owe more particularly to the epoch-making experi- 

 ments of Pasteur. 



Pasteur started from the view that cow-pox virus is an attenu- 

 ated form of small-pox virus, and proceeded to apply the experi- 

 ences attained through vaccination against small-pox to all cases 

 in which he was in possession of attenuated bacteria virus. Success 

 crowned his experiments and showed the correctness of his a priori 

 reasoning. 



He and his scholars succeeded, in the case of chicken-cholera, 

 and later in the case of anthrax, swine erysipelas, symptomatic 

 anthrax (quarter evil, black-leg), and, as we know, also in the case 

 of an affection called canine madness (which is, perhaps, a bacterial 

 disease), by the cautious inoculation of micro-organisms in various 

 degrees of attenuation, to give immunity to animals against inocu- 

 lations of material which would otherwise have been infallibly fatal 

 to them. 



Artificial immunity is usually the more perfect and durable the 

 more cautiously the protective inoculation is performed. One must 

 not employ the unattenuated virus directly after the virus in its 

 fully-attenuated form, but must proceed gradually to the former, 

 employing gradually-increased strengths. Pasteur employs two 

 " vaccines " of different degrees of virulence : the weaker of the two, 

 the " premier vaccin," generally makes the animals moderately ill; 

 the stronger, the "deuxi^me vaccin," may, under some circum- 

 stances, have fatal effects. Yet this occurs only in rare cases, and 

 generally speaking the animal gets over it without injury, and is 

 then able to receive and to support the unattenuated poison. 



It may easily be conceived that such a surprising discovery at 

 once led to a host of the boldest hopes and plans, and that the 

 practical worth of the fact was regarded as extremely great. 



