7()4 MODE OF SPREAD OF INFECTIVE DISEASES. 



mode of life. In tropical climates the remarkable dimi- 

 nution of small-pox in the rainy season may be due to 

 the easier cleansing of the body and to the increased 

 difficulty in the transport of the infective agents by cur- 

 rents of air, perhaps also to certain habits of life which 

 cannot be thoroughly appreciated. 



Local yaria- The mode of spread of scarlet fever forms a very 

 latinai n *" striking example of the effect of locality. While epi- 

 epidemioB. demies of scarlet fever have constantly occurred in 

 Europe during the present century, and many of them 

 have spread from place to place, some towns have re- 

 mained entirely free from the disease for years, although 

 they have been in communication with the infected 

 places. Thus Minister has been free from scarlatina for 

 50 years, Tuttlingen for 35 years, and Ulm for 17 years, 

 while long intervals have occurred between the epidemics 

 in Lyons and also in the whole department of Indre-et- 

 Loire.* Experiences of this kind might very readily 

 mislead us into supposing that there was some special 

 influence, proceeding from the nature of the soil or from 

 some other part in the surroundings of man, which 

 favoured the development of scarlatinal epidemics in the 

 one case and hindered it in the other ; and yet we have 

 overwhelming proof that scarlet fever only spreads by 

 contagion. Nevertheless, in spite of the purely conta- 

 gious character of the disease, differences in its local 

 distribution may quite well occur. Thus it may happen 

 that all sorts of accidents, which can scarcely be gauged, 

 may prevent the infective agent, although repeatedly 

 introduced, from causing infection ; for example, these 

 agents may only come in contact with individuals which 

 have been previously attacked, and are therefore immune ; 

 or again, the infective agents may find a difficulty in 

 obtaining a foothold in these individuals, owing to their 

 cleanly habits, and so on. In a similar manner it is 

 possible that a particular city may present better oppor- 

 tunities for a long continued immunity than another, 

 in that the population may have been largely attacked 

 during a former epidemic, or in that the spread of the 



* Hirseh, 1. c., p. 128. 



