114 PA THOGENIC BA CTERIA. 



facts later to be discussed, it may not be the true ex- 

 planation of the immunity. 



II. Experimentally acquired immunity differs from 

 the above forms in that it depends upon conditions so 

 purely artificial that they could not occur in nature. It 

 is a subject of extreme interest and furnishes us with 

 most remarkable biological puzzles. 



Some of the experimental immunities are active, some 

 are passive, some of them consist in creating resisting 

 powers where none normally existed, some in increasing 

 those that were normally possessed by the animal. 



Active Immunity — a. Inoculation. — By this term I 

 differentiate between the spontaneous infection that is the 

 common cause of accidental immunity and the inten- 

 tional experimental infection practised in the laboratory. 

 Inoculation was practised centuries ago as a means of 

 securing immunity to variola. The theory was good, 

 but the practise had very decided drawbacks. In per- 

 forming inoculation a mild case of the disease was selected, 

 at a time when no epidemic was in progress, and from 

 the variola pustule some of the matter was conveyed to 

 an abraded surface of a healthy person. The result was 

 a mild attack of smallpox, followed by perfect immunity. 

 The disadvantages were that the case might assume 

 serious aspects, and occasional deaths resulted from the 

 treatment. Further, that the inoculated individual, 

 having real variola, was a source of contagion and 

 might excite an epidemic. 



In the laboratory, inoculation is practised to bring 

 about immunity to many diseases, our knowledge of the 

 phenomena of infection being drawn upon to prevent the 

 death of the animal. It is the rule that in infecting an 

 animal a certain number of bacteria are necessary. If 

 less than this number are given, the animal shows no 

 symptoms or recovers, afterward becoming immune to a 

 much larger dose than was received. This increase of 

 resisting power is made use of in the treatment known as 

 immunization, to be described below. 



