SCIENTIFIC BASIS OF SERUM THEORY 323 



it was found a number of years ago that, by the use of weakened 

 cultures of the black-leg germ, it was possible to secure a mild 

 reaction which developed a resisting power or immunity to attacks 

 by the vigorous germs of this same disease. 



This second method of producing immunity by an injection of 

 the body of the healthy animal with a weakened culture of the 

 actual germ of the disease was never very successful in hog-cholera. 

 This is, in large measure, no doubt explained by the fact that we 

 are unacquainted with the exact definite character of the body 

 which produces this disease. We are positive that there is some 

 form of germ or virus in the blood of the sick animal, but just what 

 it is and what its exact nature is, we must confess, still remains un- 

 solved. If investigations now in progress solve this scientific mys- 

 tery it may materially change and simplify the handUng of hog- 

 cholera. 



It has been found in the case of hog-cholera, however, that in- 

 jection can be made with a full-strength culture of the cholera 

 virus without producing harmful effects, provided a dose of pro- 

 tective serum be given at the same time. This double injection 

 results in a very mild, practically unnoticeable, attack on the body 

 cells by the virus of cholera, and leaves as an after-result a perma- 

 nent protective power against this same virus which is capable 

 of producing such death-dealing results when given to the unpro- 

 tected animal. 



Active Immunity. — In these three methods of producing pro- 

 tection against disease it will be noticed that the cells of the 

 body of the animal protected have taken an active part. In all 

 these cases, whether there be an actual attack by the germs of the 

 disease or a mild attack produced by a weakened germ or a healthy 

 germ, counteracted by the presence of a protective serum, there 

 has been an actual part taken by the cells of the body. This is, 

 then, a protection acquired as the result of an active reaction on the 

 part of the tissues of the animal body, and such an immunity is 

 classed as an active immunity. 



An active acquired immunity, then, is one which is acquired by 

 the animal after birth as a result of an actual attack, or a mild 

 attack, by the germs of the disease in question. This form of 

 protection is usually long lasting in nature. In fact, it usually re- 



