766 MODE OF SPREAD OF INFECTIVE DISEASES. 



number of cases of typhoid fever increases, while it 

 influence of a diminishes as the ground water rises. In explanation 



drying zone of .. . . ,. . J ' , . , 



the soil on the ol tnis phenomenon the same iactors must be taken 

 typhokHever * n * accoun t as afforded to some extent an explanation 

 of the corresponding variation in the cholera epidemics, 

 but in the case of typhoid fever the drying of the super- 

 ficial layers of the soil at the time when the frequency 

 of typhoid fever increases comes more markedly into 

 play than in the case of cholera. As the typhoid bacilli 

 undoubtedly often reach the soil with the dejecta, either 

 when fresh or after being retained for some time in cess- 

 pools, and as they are present partly in the spore form, 

 and can thus be preserved for a long time and carried 

 by currents of air, the drying of the surface of the soil 

 or the sinking of the ground water must exert a much 

 greater influence on the spread of this disease than in 

 the case of cholera. The typhoid spores can, when a 

 dry zone is present, be very readily carried by currents 

 of air from the surface of the soil along with the dust 

 to dwellings and to various materials on which they 

 can develop, and thus the chances of infection are 

 much increased at these times. 



It is true that there is no absolute necessity for 

 assuming an exclusive influence of the soil as an ex- 

 planation of the coincidence between the frequency of 

 typhoid fever and the dryness of the soil at the end of 

 Other factors. summer and during harvest. It is quite possible that, 

 as in the case of cholera, other factors which come more 

 markedly into play at these periods such as the cha- 

 racter of the food, the large numbers of insects, factors 

 which influence the individual predisposition, &c. 

 have an influence on the seasonal distribution of 

 typhoid epidemics. In those cities where there is a 

 constant and immediate connection between the dryness 

 of the soil and typhoid fever we can scarcely go wrong 

 in ascribing the sudden increase of cases chiefly to the 

 Their multi- drying of the surface of the soil. Where, however, the 

 seasonal relation is less marked, and where we have also 

 to explain a local distribution, we must not without very 

 strc ng reason refer the explanation only to alterations 



