110 SAPROPHYTISM, PARASITISM, AND PATHOGENISM 



typhoid, plague or influenza, may leap suddenly to epidemic propor- 

 tions, spread rapidly and then subside again, to be succeeded by 

 sporadic cases which gradually diminish in numbers and in severity. 

 The bacteria causing these outbreaks appear to acquire somehow and 

 somewhere, an unusual degree of invasiveness and they spread rapidly, 

 especially in thickly settled areas, and as rapidly lose their unusual 

 activities and subside to what appears to be their usual level of viru- 

 lence. It is very probable that those strains of pathogenic bacteria 

 in general, which suddenly acquire unusual virulence are short-lived, 

 partly because their hosts perish before the microbes can escape to 

 new hosts. Not infrequently, these or similar epidemics are preceded 

 by mild, atypical disease, which may not be specifically recognized, 

 and during this initial period the bacteria may be quite widely dis- 

 tributed. 1 



1 Kendall, Boston Med. and Surg. Jour., 1915, clxxii, 851. 



