MODE OF SPREAD OF INFECTIVE DISEASES. 731 



can therefore be transmitted not only by direct contact Contagion by 

 but also by all sorts of objects which play the r6le of objecS! 3 

 transporting agents ; some are characterised by a par- 

 ticularly lengthened resisting power, and in the case of 

 these, the number of modes of transport is markedly 

 greater. In effecting the transport, and in preserving 

 the infective agents, surrounding objects are not all of 

 equal value, some act better, others worse. Porous sub- 

 stances, soil, articles of clothing, &c., appear to be 

 particularly well fitted for these purposes. 



In the preceding paragraph we have only referred to the 

 case where the objects in human surroundings act merely 

 as indifferent means of transport for the contagious 

 infective agents, and where the latter do not grow or 

 multiply on them ; these infective agents are only able 

 to multiply in the human body, and therefore belong 

 to the obligatory parasites. 



But there are also contagious infective agents which 

 can lead a saprophytic existence on the dead materials 

 in our surroundings, and must therefore be classified as 

 facultative parasites. In this case there is a multipli- Multiplication 

 cation of the sources of infection outside the body of the ive agents on 



patient, and this may go on to such an extent in our sur- 



roundings, that there are in fact more chances of infection 



by the infective agents produced outside the body than 



by direct or indirect transmission by indifferent objects 



of the infective agents given off from the patient. On the 



whole, however, the simple transport of resistant infective 



agents by various objects is in many respects of as great 



importance for the spread of a disease as when a cer- 



tain degree of multiplication is possible outside the 



body ; where the obligatory parasites are preserved for a 



long time, and are very resisting, the important influence 



of the surroundings is often as marked as when multipli- 



cation of the possibly less resistant facultative parasites 



has here and there taken place. In recent times special Slight import- 



stress has been laid on the distinction between the infective phytic g S rawth 



agents according as they belong to the facultative or to 



the obligatory parasites, or according as they multiply 



in the surroundings, or pass through them unchanged ; 



