ESSAYS ON BACTERIOLOGY. 103 



First. The immunity or protection afforded by 

 one attack of a disease is not necessarily absolute. 



Illustrations of this rule are met with in our daily 

 practice. Thus, we see those who have unquestion- 

 ably had cholera, typhoid fever or scarlatina, suffer- 

 ing from second or even third attacks. This is true 

 also of smallpox. Such experiences are exceptions, 

 rare exceptions with some diseases, but they occur 

 with sufficient frequency to warn us against too rad- 

 ical views of the value of acquired immunity, while 

 they in no way disprove the fact of such immunity. 



Second. The protection afforded by one attack of 

 a disease is not necessarily permanent. 



That the protective influence of the primary attack 

 fades with the lapse of time is, with some diseases, a 

 matter of common observation. The duration of the 

 immunity varies within wide limits. With some 

 maladies it is practically life-long; with others so short 

 as to be, to ordinary experience, hardly discernible. 

 The difficulty of defining these limits is increased by 

 the necessity of including in the calculation that pe- 

 culiar insusceptibility to certain diseases which seems 

 to be gradually and naturally acquired with advanc- 

 ing years. In this connection we must, however, not 

 forget the havoc wrought, even among the adult pop- 

 ulation, with the advent of a new disease. The Sand- 

 wich Islands were swept, as by a hurricane, by the 

 fatal breath of measles in 1848, while the Fiji Island- 

 ers fled in terror from its deadly presence. 



