460 IMMUNITY. 



that produced by vaccination, and it is found that the 

 degree of protection or immunity resulting occupies an 

 intermediate position. The corresponding general con- 

 clusion from experiments is that the more virulent the 

 organism injected, provided that the animal recovers satis- 

 factorily, the higher is the degree of immunity acquired by 

 it against that organism. Thus in developing immunity 

 of the highest degree the most virulent organisms are 

 ultimately employed. A corresponding principle, with 

 certain restrictions (vide p. 472), obtains in the case of 

 toxines. 



Immunity and Recovery from Disease. Recovery from 

 an acute infective disease shows that in natural conditions 

 the virus may be exhausted after a time, the period of time 

 varying in different diseases. How this is accomplished 

 we do not yet fully know, but it has been found in the case 

 of diphtheria, typhoid, cholera, pneumonia, etc., that in the 

 course of the disease certain substances (called by German 

 writers Anfikorper) appear in the blood, which are antagon- 

 istic either to the toxine or to the vital activity of the 

 organism. In such cases a process of immunisation would 

 appear to be going on during the progress of the disease, 

 and when this immunisation has reached a certain height, 

 the disease naturally comes to an end. It cannot, how- 

 ever, be said at present that such antagonistic substances 

 are developed in all cases. 



ARTIFICIAL IMMUNITY. 



Varieties. A number of facts regarding immunity have 

 been given in the description of the pathogenic organisms in 

 previous chapters. We shall here give a general systematic 

 description of the methods, and discuss the principles 

 involved. According to the means by which it is produced, 

 immunity may be said to be of two kinds, to which the 

 terms active and passive are generally applied, or we may 

 speak of immunity directly, or indirectly produced. 



