254 THE RELATION OF FLIES TO SUMMER DIARRHOEA 



effect upon the prevalence of flies. He states: "The immunity 

 of well-to-do infants may be explained, partly by the distance that 

 separates the sick from the healthy, and partly by the small 

 number of flies in their neighbourhood. In poor districts six or 

 seven babies may occupy the tenements of one house with a 

 common yard where flies congregate and flit in and out of the 

 open windows, themselves conveying infected excrement to the 

 milk of healthy infants, or depositing the excrement in the dustbin, 

 whence it may again be conveyed into the house by other flies. 

 Calm weather promotes dian-hoea and high winds are unfavourable 

 to the spread of diarrhoea and to the active migration of flies 

 alike. Loose soil and fissured rock, containing organic filth in its 

 crevices, favour the spread of diarrhoea and the breeding of flies, 

 whilst solid rock is unfavourable to both." Ainsworth (1909) 

 studied the relation of flies to infantile dian-hoea in Poena and 

 Kirkee, India, and by means of a yearly curve illustrates the 

 relation, which, as in the case of similar curves constructed from 

 English statistics, affords, or would appear to afford according to 

 the critics, evidence of a close relationship between flies and 

 summer diarrhoea. 



The most exhaustive epidemiological study of the relation of 

 flies to this disease has been made by Niven (1910) in Manchester. 

 He commenced to make observations in 1903, and from 1904 sys- 

 tematic captures of flies were made at selected stations. After 

 a consideration of the various factors which have been studied in 

 relation to summer diarrhoea such as soil, temperature, etc., he 

 states : " What we require for the explanation of the facts of 

 summer diarrhoea is the presence of some transmitting agent 

 rising and falling with the rise and fall of diarrhoea, the features 

 pertaining to which must correspond and explain the features of 

 the annual wave of diarrhoea. None of the facts of which we 

 have cognizance do afford such an explanation, and we come by 

 exclusion to consider the house-fly. The process of conveyance is 

 not striking and arresting as it is in military camps abroad ; nor 

 does the number of flies usually approach that observed in tropical 

 and sub-tropical countries. We are, therefore, obliged to attack 

 the question de novo, and examine such evidence as we possess to 

 see whether we may rest reasonably confident that in flies we have 



