INFECTIOUS DISEASES. 291 



origin than are the denizens of populous regions, and especially of 

 cities having commercial relations with all parts of the world. But 

 when an exotic pestilential disease is first introduced among people 

 who have previously enjoyed an immunity from it, on account of 

 their isolation, it is usually very fatal, owing to the great suscepti- 

 bility of a virgin population. This is due to the fact that there is 

 no individual immunity resulting from a previous attack, and also to 

 a relatively great race susceptibility as compared with a people among 

 whom the disease has prevailed for many years. It is evident that 

 the continued prevalence of an infectious disease in a given area will 

 have a tendency to reduce the susceptibility of the population, in 

 accordance with the laws of natural selection and survival of the 

 fittest. 



In illustration of this I may mention the comparative immunity 

 of the African race to malarial fevers, which are so fatal to Europeans 

 who visit the malarious regions of the African coast and interior; 

 and the immunity of the native (" Creole ") population of those 

 cities where yellow fever prevails as an endemic disease, as at Ha- 

 vana, Vera Cruz, and Rio de Janeiro. 



What has been said will suffice to show that the geographic dis- 

 tribution of infectious diseases is to some extent influenced by the 

 relative susceptibility of the population in various regions. The 

 prevalence of the strictly contagious diseases also depends to some 

 extent upon climatic conditions, although to a far less degree than is 

 the case in our second group, which includes diseases in which the 

 germ may multiply external to and independently of infected in- 

 dividuals. 



In general, contagious diseases are more likely to spread in north- 

 ern latitudes, and during the winter season, because the climatic con- 

 ditions lead to the aggregation of individuals in towns and in closed 

 apartments, while in southern latitudes and during the summer sea- 

 son a larger proportion of the population live in the open air during 

 the daytime and sleep in well-ventilated rooms at night. 



The influence of season upon the prevalence of smallpox, a 

 strictly contagious disease, has been referred to by numerous authors, 

 and is insisted upon by Hirsch in his Handbook of Geographical and 

 Historical Pathology. In a table contained in the monumental 

 work of Hirsch the season is given in which ninety-nine epidemics 

 of smallpox reached their height. In sixty-seven it was during the 

 cold season and in thirty-two during the warm season. The same 

 thing is shown by the mortuary statistics of various civilized coun- 

 tries. The immunity resulting from vaccination has largely in- 

 fluenced the geographic distribution of smallpox epidemics, which 

 are now almost unknown in Germany and are comparatively infre- 



