PERIODS OF INFECTIVITY. 79 



harbors in its alimentary tract for several days 

 or weeks the most virulent plague bacilli, but 

 eventually they are cleared away. In those in- 

 stances in which hereditary transmission occurs, 

 as in piroplasmosis, South African tick fever, and 

 Eocky Mountain spotted fever, the eggs may con- 

 tain large numbers of the specific micro-organisms 

 and yet give rise to apparently healthy larvae, 

 which are able to pass through the successive 

 stages into the adult form, still retaining the viru- 

 lent micro-organisms in their organs and cells. It 

 is probably by virtue of this bland relationship 

 that insects are able to figure as the carriers of 

 infection. If the latter were more virulent for 

 the insect hosts it is fair to assume that their 

 death in large numbers would minimize their role 

 as inoculators of other animals. 



In the case of the flea and plague, the insect 

 is infective immediately or soon after it has in- 

 gested blood, but the period of infectivity is lim- 

 ited to a few days (Verjbitski) or at the most a 

 few weeks (the Indian Plague Commission). This 

 has been referred to above as acute infection of 

 the insect. Mechanical transmisssion is a term 

 which has been used to signify the pure mechan- 

 ical conveyance of micro-organisms by an insect 

 from a diseased to a healthy animal. The role of Periods of 

 the flea in carrying plague may be simply a me- 

 chanical one, particularly when it transfers the 

 disease immediately after its infection. But, inas- 

 much as proliferation of the bacillus seems to 

 occur in the flea, and since his infectivity may last 

 for three weeks or more, proliferation may be re- 

 sponsible for the later infectivity. In this event 

 the transmission depends on biological as well as 



