80 INFECTION AND IMMUNITY. 



Mechanical mechanical factors. Transmission of this type 

 probably demands a high degree of virulence and 

 infectivity on the part of the micro-organisms, 

 necessitating the introduction only of very small 

 numbers. One of the conclusions of Verjbitski is 

 that "animals could not be infected by the bites of 

 fleas and bugs which had been infected by animals 

 whose own infection had been occasioned by a cul- 

 ture of small virulence, notwithstanding the fact 

 that the insects may be found to contain abundant 



plague microbes." 10 



Another condition is represented in Eocky 

 Mountain spotted fever, in which the insects are 

 infective not onty immediately or soon after suck- 

 ing diseased blood, but also for an indefinite sub- 

 sequent period. In this instance the tick under- 

 goes generalized infection, as previously men- 

 tioned. It is possible that the conveyance which 

 takes place immediately after infection is of the 

 mechanical type, but this cannot be true of the 

 later transmissions, nor of those by the members 

 of the following generation. In the latter we have 

 to do with biological transmission, which in these 

 two cases depends on proliferation of the micro- 

 organisms and a general invasion of the tissues of 

 the insects. The term biological transmission''* 

 was originally applied to those cases in which the 

 micro-organisms (protozoan) undergo a compli- 

 cated sexual cycle of development in the insect, as 

 in malaria, but the principle remains the same 

 with the bacterial diseases, although the method 

 of proliferation is a simple one, consisting merely 

 of fission. 



10. Jour. Hygiene, 1908, viii, 205. 



