DOUBLE OR SIMULTANEOUS METHOD OF TREATMENT 383 



and get them off to market before the protection given them by 

 the serum had run out. 



On the other hand, in the case of sows and boars that it is 

 desired to keep over for another season as brood stock, it woukl 

 be much more desirable if the treatment could be so arranged as 

 to make one injection sufficient to give protection throughout the 

 lifetime of the animal. This is just what we are able to do by 

 means of the double treatment. 



In discussing immunity we found that immunity was a resist- 

 ance to disease, or protection against disease, which was developed 

 as the result of an actual attack of the disease, or might be given 

 by injecting into the body of the animal some substance which 

 carried with it bodies that would protect the animal against the 

 special disease from which immunity was desired. Now an actual 

 attack of cholera is usually fatal, and if we were to depend upon 

 protection obtained in this way we would have very few immune 

 animals, as most of them would die during the attack of the disease. 



It has been found, by careful experiments, that we can take 

 the pure virus of cholera and inject it under the skin or into the 

 muscles of an animal and produce a fatal attack of the disease 

 within a week. An example of this is seen in the manufacture 

 of serum where young shoats are injected with virus blood and 

 allowed to get sick, and are then slaughtered just before they die 

 of the disease, in order to get the virus blood necessary to inject 

 immune hogs to produce a hyperimmunity that will give us a 

 blood from which serum may be made. 



If we take the same dose of virus which we inject into these 

 unprotected pigs and give it to an animal that is protected by an 

 immunity resulting from an attack of the disease, there are no 

 symptoms produced. Even a thousand times the usual death- 

 producing dose of virus we find only stimulates the development 

 of more immune bodies in one of these protected animals. 



We have also seen that if an animal be given a dose of serum 

 and placed in the same feed lot with sick cholera hogs the pro- 

 tected hog will not develop the disease. 



Now we can go even a step further, and find that if we inject 

 a dose of the cholera virus and at the same time inject into the 

 animal a sufficient dose of a good strong serum, the animal will 



