ARTIFICIAL IMMUNITY. 425 



we find that the degree of protection is less complete and 

 lasts a shorter time than that produced by the natural 

 disease. Again, inoculation with lymph from a smallpox 

 pustule produces a form of smallpox less severe than the 

 natural disease but a much more severe condition than 

 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 employed. 

 A corresponding principle, with certain restrictions (vide 

 p. 438), 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 Antikorper) are found to appear in the blood, which 

 are antagonistic either to the toxine or to the vital activity 

 of the organism. In these 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, 

 however, be said at present that such antagonistic substances 

 are developed in all cases, as there are other means by which 

 the spread and multiplication of the organisms may come 

 to be arrested. 



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 



