INTRODUCTION 5 



bacteria and ova are present in the faeces of infected persons, flies 

 jindei- S2iitable conditions, mainly by infecting food substances, 

 probably aid in the dissemination of the diseases these organisms 

 produce. 



It is scarcely necessary to point out that flies can only act 

 as carriers of disease germs after they have come into contact 

 with suitably infected materials, and that their opportunities for 

 infecting themselves are in proportion to the care exercised by 

 the community in removing such materials or rendering them 

 harmless. 



In military camps, where the conditions are often very 

 favourable, typhoid fever seems to be disseminated by them, and 

 the evidence relating to the part they may play in the spread of 

 other diseases, such as infantile diarrhoea in temperate climates, 

 and cholera, ophthalmia, and yaws in tropical countries, is very 

 suggestive. Direct proof is, however, still almost lacking, and 

 we have no reliable information concerning the extent to which 

 they are responsible for the spread of any disease. 



The larvse of non-biting flies sometimes infest man, living 

 under the skin or in wounds or natural cavities or in the intestinal 

 canal. 



It is certain that the house-fly is a potential disease carrier 

 and a constant frequenter and disseminator of filth, but much 

 remains to be done before Howard's name ' typhoid fly ' or 

 Hewitt's generalization can be completely justified. To both 

 these investigators the greatest credit is due not only for the 

 work they have done, but for the manner in which they have 

 stimulated enquiry, by persistently bringing the subject to notice. 

 Both, approaching the subject from the entomological standpoint, 

 have based their conclusions in regard to disease mainly on 

 evidence of an epidemiological character, and have apparenth' 

 accepted the bacteriological evidence almost without criticism. 

 From the bacteriological point of view, however, while the 

 evidence relating to the carriage of pathogenic bacilli by experi- 

 mentally infected flies is fairly conclusive, that relating to the 

 presence of these micro-organisms in ' wild ' flies is far from 

 complete. The records are few, several of them are old, and only 

 a small proportion reliable. 



