CHAPTER XXI 



THE CARRIAGE OF TYPHOID FEVER BY FLIES 



Of all infectious diseases the conditions incidental to this 

 disease are most favourable tor the transference of infection by 

 M. domestica, and it is no doubt on this account that the greatest 

 attention has been paid to the role of house-flies in the dissemina- 

 tion of this disease. The chief favourable condition is that the 

 typhoid bacillus occurs in the stools of typhoid and incipient 

 typhoid and " carrier " cases. Human excrement attracts flies not 

 only on account of its moisture but as suitable food for the larvae. 

 The infected excrement is often very accessible to flies, especially 

 in military camps, as will be shown shortly ; the flies also frequent 

 articles of food and not infi-equently the moist lips of man. Such 

 are the conditions most suitable for the transference of the bacilli, 

 and it is on account of the frequent coincidence of these conditions 

 that flies can play, and have played, such an important role in 

 the dissemination of this disease among communities, in spite of 

 the fact that the typhoid bacillus cannot survive desiccation, 

 which I think is an argument against its being carried by dust. 

 The danger incident to the carriage of this non-spore-bearing 

 bacillus in the digestive tract of the fly probably exceeds that 

 resulting from direct carriage on the appendages and body of the 

 insect. 



The possibility of flies becoming infected with Bacillus typlLOSus 

 is increased with the frequent accessibility of flies in numbers to 

 infected material. This fact is most clearly demonstrated in 

 military and construction camps, and during typhoid epidemics in 

 the slum and insanitary portions of towns and cities. 



