122 PATHOGENIC BACTERIA. 



solution recommended by Behring, or by diluting it, 

 attenuating it by heat, or by using less virulent cultures, 

 etc. The same dose is repeated, and after some days, 

 if the reaction has been mild, a larger dose can be 

 given. In a week, more or less, according to circum- 

 stances, a still larger dose may be injected and so on 

 until, by proper careful management through a sufficient 

 length of time, the animal may be so accustomed to the 

 toxin as to endure, without symptoms of inconvenience, 

 hundreds of times the originally fatal dose for his kind. 



The condition is one of habituation or tolerance, and 

 leads, of course, to a very unnatural degree of immunity. 

 When we come to study this state of forced immunity we 

 find ourselves encountered by a most interesting series of 

 paradoxes and phenomena. 



No visible change is discerned during the process, the 

 animal remaining well and strong throughout. The 

 toxins injected at the regular periods are endured without 

 inconvenience, and to all appearances the process of im- 

 munization might go on indefinitely. Unexpectedly, 

 however, the first obstacle and paradox appears. No 

 matter how carefully the immunization process has been 

 carried on, there may come a time when further increase 

 will develop most unexpected symptoms, entirely charac- 

 teristic of the disease, and, in my experience, invariably 

 fatal. This condition was long ago pointed out by 

 Behring, who showed that the injudicious or too rapid 

 increase of the toxin doses in immunization threw the 

 animal into a state of hypersensitivity in which it suc- 

 cumbed to doses easily endured before. This hypersen- 

 sitivity is not affected by the fact that the blood of the 

 animal contains antitoxin, and is quite as likely to come 

 on in highly antitoxic animals as in others. 



The duration of acquired immunity is very variable, 

 according to the particular conditions by which it has 

 been induced. Accidentally acquired immunity, as in 

 the case of yellow fever, may be so perfect and perma- 

 nent as to endure throughout the remainder of the indi- 



