GREAT EPIDEMICS ASIATIC CHOLERA. 283 



tionable and contradictory results. In Europe, high re- 

 gions have generally been spared, but the epidemic has 

 raged violently on the plateaus of Mexico and the summits 

 of the Himalayas. If localities overlying granite and other 

 solid rocks have seemed to enjoy special immunity, as Pet- 

 tenkofer has proved, cases are known, like that of Helsing- 

 fors in 1849, in which those parts of the town built on 

 granite were decimated, while the marshy parts and those 

 near the shore remained exempt. Some countries, such as 

 WUrtemberg, some cities, Lyons for instance, have hitherto 

 escaped the attacks of the pestilence almost wholly, with- 

 out any assignable reason. What is more indisputable is 

 the fact that the collection of multitudes of people facilitates 

 the development of the disease. Armies in the field, popu- 

 lous cities, make a sort of centre from which it radiates. 

 Thus the war in Poland in 1831 seems to have been the cause 

 of the rapid spread of the cholera in Europe. We know 

 no instance of a rural population swept by the epidemic 

 where there was not a town in the neighborhood which 

 had first suffered from its effects. In the towns, the most 

 closely-built and unwholesome quarters are attacked and 

 affected'more severely than the others. In a word, the chol- 

 era has a special affinity for large collections of human be- 

 ings ; in them it concentrates, and through them it spreads. 

 Observed facts are decisive in this respect, and no argument 

 could prevail against the accumulated evidence. The close 

 study of epidemics shows that we must attribute the more 

 or less rapid spread of cholera beyond the centre of its ori- 

 gin neither to winds nor to water-courses, nor to supposed 

 miasmatic emanations, but it must be attributed to pilgrim- 

 ages, fairs, movements of troops, and similar changes of 

 place by masses of men. Single travelers in good health 

 have, as may well be imagined, very few chances of carry- 

 ing the disease with them from an infected country to a 

 healthy one ; but travelers in crowds, among whom there 



