MODE OF SPREAD OF INFECTIVE DISEASES. 76 1 



power of the sources of infection, on account of the easy 

 transportation of the infective agents, and on account 

 of the readiness with which the body is attacked, can he 

 influenced with extreme difficulty by other prophylactic 

 means. It is further a disease which is very specially 

 suitable for a protective inoculation on account of the 

 peculiar certainty and persistence of the immunity. On Advantages of 

 the other hand, in the case of the majority of other in- p 

 fective diseases, it is decidedly more rational to aim at means - 

 the destruction of the infective agents, and the preven- 

 tion of their spread or invasion, than to rely on an un- 

 certain protective inoculation. That in the case of 

 rabies suitable prophylactic rules deserve by far the 

 greatest confidence is shown most distinctly by the con- 

 dition of those countries in which a high tax on dogs, 

 stringent rules against stray or possibly rabic animals, 

 and under certain circumstances compulsory muzzling, 

 have been already introduced ; statistics show that in 

 these countries (Prussia, Bavaria, &c.) the deaths from 

 rabies in dogs have diminished to a marked degree, and 

 in man have practically disappeared. 



6. The Local and Seasonal Predisposition to Infective 

 Diseases. 



From the foregoing considerations as to the mode of irregular 

 spread of the infective diseases, we may draw the con- t 

 elusion that this distribution cannot occur either equally diseases 

 over a large area or in an unbroken course as regards time, 

 but that seasonal and local variations must appear and 

 exert an influence on the occurrence of these diseases. 



In none of the infective diseases is the spread of the 

 infective agents like that of a gas which diffuses itself 

 equally in all directions, and over great distances ; on 

 the contrary, the effective agents, even in the most 

 favourable cases, only spread equally from one centre 

 over a small distance. Even the most typical con- 

 tagia, viz., the exciting agents of the acute exanthe- 



