24 



On the other hand, we have excellent a priori reasons for 

 supposing that flies could transmit the infective agent of 

 diarrhoea. Anyone familiar with the domestic manage of 

 the average working man on a hot summer day, with the 

 baby sick with diarrhoea and other small children to care 

 for, must realise that the opportunities afforded for fly 

 transmission are adequate enough. 



It seems to me, therefore, that before abandoning the 

 view that diarrhoea is a bacterial infection or making the 

 gratuitous assumption that a long spell of warm weather is 

 necessary to awaken pathogenicity, one must carefully 

 examine the adequacy of the hypothesis that the depend- 

 ence of the epidemic on accumulated temperature is to be 

 accounted for by the operation of this factor upon the means 

 of transmission. 



We will now see whether this interpretation fits the rest 

 of the facts. The greater steepness of the rise of the 

 diarrhoea curve is consistent with this view, for at first the 

 opportunities for a fly to transmit the infection are small, 

 but as the cases increase opportunities increase likewise. 

 As has been pointed out by Hamer and Peters, if variation in 

 the number of fly transmitters of an infective agent existing 

 in the stools of patients were the sole factor concerned, the 

 curve for diarrhoea cases should shoot up beyond and come 

 down later and more slowly than that representing the 

 number of flies, because the opportunities afforded to flies to 

 pick up the infective agent are increasing pari passu with 

 the development of the epidemic. This is, however, not what 

 happens. On the contrary, the epidemic is arrested while 

 flies are numerous, and declines more quickly than the fly 

 population. We are therefore confronted with the question : 



What Arrests the Rise in the Epidemic Wave ? 



Exhaustion of susceptible material has been suggested. 

 This obviously plays a part in every epidemic, and the 

 observations of Peters (1910) in Mansfield where, under 

 similar conditions, in one section of the town the epidemic 

 was nearly finished, whilst in another it was beginning 

 afford a demonstration that it is a factor of importance. As, 

 however, small epidemics in cool summers exhibit the same 

 feature it cannot be entirely responsible for the arrest. 



Another factor which must be taken into account is the 

 atmospheric temperature at the time. A glance at the 

 charts, Figs. 7, 8, 9, 10, and 11, in which the weekly mean 

 temperature is also shown, suggests that this is playing an 

 important part, for the decline of the epidemic occurs 

 immediately upon a considerable drop in mean temperature 

 and a diminution of fly prevalence follows shortly afterwards. 



The influence of atmospheric temperature appears to be 

 more marked upon the number of infections than upon the 



