SPIRILLUM CHOLERA ASIATICS. 455 



numerous or are entirely removed, or because the usual 

 paths of transport have become too few, or because the 

 exposed individuals are immune against the infection. 



Accordingly those external factors which can act to a 

 great extent on the sources of infection, on the paths of 

 distribution, or on the exposed individuals, will probably 

 furnish an explanation of the local and seasonal dif- 

 ferences in the spread of cholera ; and hence we must 

 study more closely these external factors. We find these 

 factors partly in meteorological conditions, partly in the 

 soil, and partly in the habits and customs of different 

 races. The most important are the following : 



1. Meteorological influences. High temperatures 1. Meteoro- 

 approaching the optimum of the temperature of the infliaences. 

 comma bacilli might favour the spread of the disease by 

 enabling the comma bacilli to multiply as saprophytes, 

 and by thus multiplying the sources of infection. And Temperature, 

 the less energy of tissue change, and the less resisting 

 power of the body by which excessively high temperatures 

 are apt to be accompanied, appear to increase the indi- 

 vidual predisposition to a certain degree. On the other 

 hand, epidemics are by no means absent in winter, be- 

 cause these favouring influences of temperature are not 

 so important that they cannot be completely replaced by 

 other predisposing factors. It must also be remembered 

 that in winter we usually employ in our immediate sur- 

 roundings artificial heating, and thus produce tempera- 

 tures which are quite sufficient to enable the comma bacilli 

 to multiply ; and that on the other hand, in summer and 

 in warm climates sources of infection which may be pre- 

 served for a long time at a low temperature are more 

 readily rendered inert by drying, or by the quicker 

 growth of saprophytic bacteria. On the whole, therefore, 

 it is only rarely that temperature has a decisive influence 

 on the spread of cholera. 



Very great dryness of the air must render the trans- Moisture of 

 mission of the disease more difficult, in that it occasions 

 rapid death of the comma bacilli in the sources of infec- 

 tion. Cholera linen, soil contaminated with dejecta, &c., 

 are under these circumstances only infective for a very 



