26 



Factors Contributing to Decline of Epidemic. 



We have, then, two factors which may contribute in 

 varying degree to decline of the epidemic : 1. A fall in tem- 

 perature diminishing (a) the activity and number of 

 supposed transmitters ; () the dose of infection which the 

 child ingests owing to the effect upon the rate of multipli- 

 cation of the infective agent. 2. The exhaustion of the 

 more susceptible individuals. 



Sometimes one, sometimes another, of the above factors 

 appears to predominate. For instance, we have the fact, to 

 which attention was drawn by Peters (1909), that in a much 

 hotter climate, such as Australia, the epidemic often declines 

 notwithstanding the fact that the temperature may continue 

 to rise for several weeks. In Fig. 12 I have plotted from a 

 paper by Stawell (1899) the monthly mean temperature 

 and monthly diarrhoea mortality in Melbourne for the years 

 1895-96. In this city, where winter and summer succeed 

 one another without the long intervening periods we have 

 here, the epidemic lasts for six months instead of six weeks. 

 It will be noticed that in 1895 the temperature continued to 

 mount for two months after the summit of the epidemic. In 

 this case the decline cannot be due to diminished dose of the 

 poison owing to slower rate of bacterial growth, but must be 

 attributed to exhaustion of susceptible individuals or to the 

 effect upon the presumed transmitting agents, the flies. In 

 the absence of definite quantitative observations I cannot 

 speak dogmatically, but as far as my memory serves me I 

 think the number of flies diminishes in Melbourne, as it 

 does in Washington, during the hot weather. A monthly 

 mean temperature of 70 in Melbourne indicates that there 

 were a good many days with a maximum shade temperature 

 of over 100, which in that locality is associated with 

 extremely dry north winds. Such meteorological conditions 

 are very fatal to flies both in the larval and imago stage. 

 On hot wind days flies have to seek shelter out of the sun or 

 they rapidly become desiccated. For the same reason they 

 succumb on an exposed window pane, and large numbers of 

 dead flies accumulate on the window frame. 



Relation between Ifly -prevalence and Diarrhoea Cases in Space. 



No quantitative observations upon the relation between 

 number of flies and the distribution of cases of infantile 

 diarrhoea have, as far I am aware, been made, but I may 

 mention a few impressions by different observers for what 

 they are worth. Nash (1909), who, whilst medical officer of 

 health for Southend, made during 1904 spot maps of the 

 epidemic there, says: "The great majority of the deaths 

 from diarrhoea were shown to have occurred in streets in 



