156 SUMMER DIARRHCEA 



thermometer ^ Flies stand, however, in a more intimate relation 

 to deaths from diarrhoea than does the 4 ft. thermometer. 



7. — Up to the present sufficient data have not been collected 

 to establish or disprove this hypothesis, but the known facts 

 from districts in Manchester lend a general support to it. The 

 data could be collected without great difficulty by planting 

 a sufficient number of traps in different districts of sufficient 

 size. 



8. — Up to the present no other available hypothesis seems 

 capable of explaining the course of summer diarrhoea. 



Niven (p. 174) sums up the analysis of his investigations as 

 follows : 



" Summer diarrhoea is an infectious disease. This is shown 

 by the course of the annual wave, by the manner of its incidence 

 on the different sanitar}^ districts of Manchester, and by the 

 history of individual cases. The summer wave is not due to 

 dust, nor is it conditioned by any growth of bacteria in or on 

 the soil. There is nothing to support the view that the infective 

 organisms are of animal origin, and the connection between 

 privy-middens goes far to prove the contrary. The disease 

 becomes more fatal only after house-flies have been prevalent 

 for some time, and its fatality rises as their numbers increase 

 and falls as they fall. The correspondence of diarrhoeal fatality 

 is closer with the number of flies in circulation than with any 

 other fact... .Certain facts in the life-history of the fly throw light 

 on discrepancies arising in the decline of flies and cases. The 

 close correspondence between flies and cases of fatal diarrhoea 

 receives general support from the diarrhoeal history of sanitary 

 subdivisions of the Manchester district. The few facts available 

 for the study of the correspondence of flies and fatal cases in 

 different subdivisions, in the course of the same year also lend 

 support to this view. No other explanation even approximatel}' 

 fits the case." 



Hamer made observations, somewhat similar to those of 

 Niven, in various parts of London in 1907, 1908 and 1909, on 

 a larger scale. He, however, caught the flies in the neighbour- 

 hood of refuse and inanure depots, where flies are bred, and 

 1 For a clear discussion of the ' varying seasonal facts,' see Niven, pp. 163—166. 



