462 



SPIRILLUM CHOLERA ASIATICS. 



may be influenced by the fact that a larger or smaller 

 proportion of the individuals have been rendered 

 immune for a time by a previous attack of cholera. As 

 has been previously stated, we must assume that even 

 the mildest attack can produce this immunity, so 

 that after a considerable epidemic a relatively high per- 

 centage of the population has been attacked. This 

 circumstance must diminish to a great degree the 

 chances in favour of the spread of a second epidemic ; 

 and in India, where there are almost always insuscep- 

 tible districts which have been recently attacked along- 

 side of the susceptible ones, the mode of spread of the 

 epidemics must consequently show peculiar interruptions 

 and leaps. Koch has called attention to the fact that 

 by this acquired immunity of certain tracts we can 

 understand the remarkable distribution of cholera when 

 it spreads from one of the large pilgrim resorts as a 

 centre ; the line which the disease takes does not 

 correspond to all the radii, although all have undoubtedly 

 been brought in contact with cholera patients ; the 

 cholera spreads only in those directions where this 

 acquired immunity is wanting. 



Differences in 

 the effect of 

 the first case 

 of cholera. 



As the result of these numerous influences, varying 

 according to place and season, variations in the epidemic 

 spread of cholera are the more readily produced because 

 the disease is not always carried from one individual to 

 another, but often attacks a large number of individuals 

 at the same time. Hence the differences in the effect 

 in cases where the chances in favour of its spread are 

 plentiful, and in those where they are few, are still 

 more strikingly manifested. "We have only to consider 

 how completely different the mode of spread is according 

 to the circumstances under which the first case of 

 cholera is introduced into a town. In the one place the 

 patient may be nursed in a well-to-do family, or in a 

 suitably arranged hospital with trained nurses ; in the 

 other place the first case may occur in a narrow, poor 



