I50 SUMMER DIARRHOIA 



certainty, and the mode of spread is uncertain. There can be 

 little doubt that direct contact, infected food, especially milk, 

 whether infected before reaching the house or within it, and 

 carelessness in dealing with soiled articles, all tend to spread the 

 disease, but in this chapter the influence of flies only will be 

 considered. 



In considering the relationship of flies to epidemic diarrhoea 

 we have two sets of data on which to base an opinion : (a) epi- 

 demiological evidence and (d) bacteriological analyses. 



(a) Epidemiological evidence. 



Many papers have been published on the epidemiology of 

 epidemic diarrhoea, but definite statistical evidence in regard to 

 the relationship of flies to the disease is rare. Niven (1910) has 

 however studied this subject with great care during a series of 

 years, 1903 — 1909, in Manchester, and has described the results 

 of his investigations in an admirable paper, which is extensively 

 quoted in this chapter. 



As Niven rightly remarks : " it is a great advantage, in 

 forming conclusions as to the explanation of facts, to deal with 

 areas all the characters of which are quite familiar to the reasoner. 

 The atmosphere, the temperatures of the soil, and the rainfall 

 are approximately the same throughout" (p. 132). 



Though a few cases are recorded week by week throughout 

 the year the disease is not much in evidence until the annual 

 summer outbreak which never occurs before June. In winter, 

 spring and early summer the mortality is low, and on investiga- 

 tion of a number of individual cases "infection from person to 

 person becomes highly probable in a considerable proportion of 

 the cases, the agency being left vague." During the summer 

 outbreak, however, the mortality is often high. 



"We may take it as proved that there is an intimate relation 

 between the storage of excreta in privy-middens and a high 

 diarrhoeal mortality." The social condition of the population 

 has also much influence on the diarrhoeal death rate, owing 

 chiefly to ignorance and carelessness on the part of many 

 mothers. 



