MODE OF SPREAD OF INFECTIVE DISEASES. 735 



kofer, is irreconcilable with the results of the more 



recent experimental investigations with regard to the 



specific exciting agents of typhoid fever and cholera. 



We see that both in typhoid fever and cholera the living 



infective agents of these diseases are given off in large 



numbers in the dejecta of the sick, and there is abso- 



lutely no reason why they should not transmit the 



disease to a healthy predisposed individual, either by 



immediate contact, or through the intervention of some 



object. Pettenkofer assumes that these agents as they Excretion of 



come from the sick are not ripe, and are not able to 



cause infection ; and he holds that they must first ac- infection by 

 quire their infective properties in a suitable soil outside 

 the body. This idea of a specific transformation of the 

 infective agents in the soil could, however, only be held 

 so long as we were completely in the dark as to the nature 

 of the infective agents ; it is no longer admissible in the 

 present state of our knowledge as to the biology of the 

 infective agents of typhoid fever and cholera. And be- 

 sides, we have definitely ascertained from experiments on 

 animals, as well as from involuntary experiments on 

 man, that there is no necessity for a transformation of 

 the infective agents after they are given off by the sick 

 in order to enable them to cause infection. 



The possibility of the spread of these diseases by Role of the 

 contagion, and its occasional occurrence, must there 

 fore be looked upon as undoubted facts. It is true that f both 

 the surroundings play a very marked part in the natural 

 distribution of these diseases, but they do so chiefly by 

 preserving unaltered the infective agents given off by 

 the sick, and bringing them in contact with predisposed 

 individuals. Among the numerous substrata and 

 objects in our surroundings, the soil appears particularly 

 well adapted for such a preservation, and therefore in 

 many cases it plays a prominent part as a means of 

 transport, To a less degree the spread of these diseases 

 is also influenced by a multiplication of the infective 

 agents outside the body on nutritive materials, on the 

 surface of the soil, &c. ; but in our climate this ecto- 

 genous development and multiplication of the typhoid 



