MODE OF SPREAD OF INFECTIVE DISEASES. 



or with infectious objects. The number of infections 

 caused by them is under certain circumstances much 

 greater and more widely distributed. That all the 

 diseases belonging to this group do not in practice show 

 an equally high degree of contagiousness, that, for 

 example, the acute exanthemata and typhus fever are 

 extremely contagious, while in typhoid fever and tuber- 

 culosis the danger of contagion is much less, is explained 

 by the other conditions necessary for infection ; for ex- 

 ample, by the situation of the seats of invasion and by 

 the individual predisposition. These two conditions 

 are referred to more in detail in the following pages. 



The detachment of the infective agents from their Conditions for 

 substrata and their passage into the air occurs from the air" 81 

 dried excreta and the various objects to which they 

 adhere from clothing, from the flooring, from the sur- 

 face of the soil. In all cases complete dryness of the 

 surface is a necessary condition for the detachment of 

 the infective germs; hence in the soil this only occurs 

 when a drying zone is present, and when the most super- 

 ficial layer is in a state of dust. 



It frequently happens that the same mode of transport indirect trans- 

 does not carry the infective agents directly from the source P< 

 of infection to the point of invasion, but that it carries 

 them in the first place to another means of transport. 

 Thus, for example, the detached infective agents may be 

 carried by currents of air to articles of food, and from 

 thence reach the exposed individual. 



3. The Seats of Invasion. 



Before we can arrive at definite ideas as to the im- Do the skin 

 portance of the seats of invasion for the spread of the 

 infective diseases, we must decide the question whether 



the bacteria can penetrate the various surfaces of the the entrance 

 body, the skin, and especially the mucous membranes, 

 when these are in a normal condition ? 



