26o SUMMARY AND CONCLUSIONS 



given time, and owing to the long incubation period of some of 

 the diseases under consideration the infected flies may have 

 disappeared before the disease is recognized and investigations 

 started. Suitable opportunities for investigation must however 

 occur from time to time, and if these are utilized definite and 

 reliable information on the subject will be obtained. 



Apart from definite bacteriological proofs, other evidence 

 clearly points to flies being the carriers of infection in certain 

 outbreaks, in which the other principal sources of infection have 

 been carefully excluded (Chapter Xll). 



B. typhosus is said to have been isolated from ' wild ' flies 

 associated with outbreaks of typhoid fever on several occasions, 

 and experimentally flies can carry and distribute this bacillus 

 for several days. There is some evidence that flies are factors in 

 causing the autumnal increase in the disease in this country, but 

 it is unlikely that they play an important part in well-sewered 

 towns. On the other hand, the evidence is very strong that they 

 are the dominating factor in the dissemination of the disease in 

 military and other camps, and in stations in the tropics. In 

 such places infection is mainly acquired from the trenches in 

 which night-soil is insufficiently buried, or from fa;cal matter 

 deposited in situations to which flies have access. The infected 

 material is derived from ' carriers ' or incipient or mild cases of 

 the disease (Chapter xill). 



In temperate climates epidemic diarrhcea is the most im- 

 portant disease flies are supposed to transmit. It is undoubtedly 

 true that unlimited opportunities are afforded to flies of conveying 

 presumably infected material from the intestinal discharges of 

 patients to the food of healthy infants, and also that there is a 

 close relationship between the curves graphically illustrating the 

 rise and fall of the annual outbreak of epidemic diarrhcea and 

 the rise and fall of the annual prevalence of flies. In the hot 

 summer of 191 1 flies were numerous and epidemic diarrhoea 

 ver}^ prevalent, but in the cold and wet summer of 191 2 flies 

 were few and epidemic diarrhoea uncommon. Evidence of this 

 nature, whilst very suggestive, is not altogether conclusive. Un- 

 fortunately, though the disease is admitted to be infectious, the 

 causative organism has not been identified with certainty. In 



