MODE OF SPREAD OF INFECTIVE DISEASES. 765 



disease from the first case may be rendered difficult on 

 account of certain habits of the population, such as 

 the mode of treatment of the sick (inunction of fat), 

 special attention to cleanliness, &c. It is, however, 

 evident that even when a place has remained free 

 from the disease for several years, although repeated 

 epidemics have occurred in its neighbourhood, we are 

 not justified in asserting that this place possesses a 

 permanent immunity, due to a direct influence of the 

 nature of the place on the infective agents ; on the con- 

 trary, all these places have ultimately been found to be 

 susceptible as soon as the effect of the previous epidemic 

 has disappeared, and the chances against the attack of 

 the infective agents have become less favourable. 



A priori we may, however, expect that these local and Greater local 

 seasonal variations will be more marked in the case of 



those infective diseases which are not so extremely con- J he . less cn- 



v tagious 



tagious as the acute exanthemata, but in which the diseases. 

 distribution is much more dependent on external acci- 

 dents. This will evidently be the case where the sources 

 of infection, the modes of transport, and the seats of 

 invasion are limited, and where a marked spread of the 

 infection can only result when there is a certain concur- 

 rence of these circumstances. The chief of these diseases In cholera. 

 are those which are occasioned by the contagious facul- 

 tative parasites, in which the number of the sources of 

 infection increases or diminishes under the influence of 

 external conditions, the modes of transport vary in their 

 value, and the seats of invasion become accessible or 

 inaccessible. The numerous influences which come into 

 play in these cases have been subjected to a more accu- 

 rate analysis in the chapter on cholera (page 443), and 

 from the description which has been given there we may 

 readily understand the manifold character of these influ- 

 ences and the applicability of the facts to other diseases 

 which show a similar mode of distribution. 



In many towns a regularly recurring seasonal varia- in typhoid 

 tion is still more marked in the case of typhoid fever fever ' 

 than in cholera ; thus during the dry harvest months, 

 and when the level of the ground water is low, the 



