MODE OF SPREAD OF INFECTIVE DISEASES. 763 



affections, that there can be a general distribution of the 

 disease without distinction of person. In every other 

 case we meet with a great number of variations. 



In the great majority of these variations it is unavoid- Laws as 

 able that we here and there come across apparent natural ie 



laws ; one locality will be more frequently attacked than 

 another, one season of the year will cause an increase in 

 the number of cases of the disease, another will cause a 

 diminution, and so on. Such recurring local and sea- 

 sonal variations are in their essence only a further result 

 of the numerous concomitant accidents, and one cannot 

 from small numbers deduce laws which justify the 

 assumption of a special local or seasonal influence. 



Here and there, however, the variations apparently 

 follow more distinct laws, and these point to external 

 influences, varying with place and season. This is a 

 priori intelligible if we consider in how marked a 

 degree the number and resisting power of the sources of 

 infection, the facility of transport, and the state of the 

 seats of invasion are dependent on causes varying with 

 place and time. Even in those diseases which are ex- In obligatory 

 tremely easily transmitted, <?.#., the acute exanthemata, pl 

 we can at times, though by no means always, recognise 

 regularly recurring seasonal or marked local variations. 

 Thus the statistics of small-pox epidemics show that the Seasonal 

 disease usually increases markedly in the cold months small-pox 

 and diminishes in summer ; and that, further, in warm e P ldemic? - 

 climates there is a marked increase in the dry season, and 

 an almost complete disappearance in the wet months.* 

 One can scarcely go wrong in attributing the regular 

 increase of small-pox in winter to the greater amount of 

 time spent at home, to the more intimate association of 

 individuals, to the wearing of the same clothing for some 

 time, and to the diminished cleansing of the skin, thus 

 rendering the transport of the infective agents and their 

 preservation at the seats of invasion much easier than 

 in summer when all these favouring factors are absent. 

 These differences are of course more or less marked in 

 different countries, according to the climate and the 



* Hirsch, Handbuch der histor. geogr. Path.. 2 Aufl., i., p. 105. 



