MODE OF SPREAD OF INFECTIVE DISEASES. 747 



developing in folds and recesses of the mucous membrane, 

 and not be readily removed with the mucus or overgrown 

 by saprophytes. In such situations the parasites might 

 be able to multiply to a certain extent, and then lead to 

 a production of toxic materials which would be sufficient 

 to destroy the neighbouring cells, and thus allow them 

 to penetrate more deeply into the tissue. Such penetra- 

 tion through normal skin and mucous membrane must 

 be assumed, more especially in the case of the infective 

 agents of the acute exanthemata, which often attack very 

 numerous individuals. 



Other infective agents, on the contrary, probably require Or the infec- 

 minute lesions or solutions of continuity of the mucous 



membrane in order to obtain a firm footing in the tissue, lesions at the 

 and multiply at first at the cost of the dead cell material, a ion. 

 and spreading from that point carry on the struggle 

 against the living cells. In all cases, however, the 

 presence of such minute injuries favours the entrance of 

 all infective agents and increases the individual predis- 

 position (see below). 



The fact that a specific and exclusive seat of invasion importance of 

 is necessary for each infective agent evidently is of great * of n t^ e of ?nva 

 importance as regards the mode of spread of the infective sion for the 

 diseases. For the transmission of this group of diseases 

 it is evidently not sufficient that the means of transport 

 should carry the infective agents from the source of 

 infection to some part or other of the body, but 

 it is also necessary that they should be brought to 

 a particular situation. In the case of cholera and 

 typhoid the disease must therefore occur chiefly as the 

 result of swallowing infected water and infected food ; it 

 is possible that the germs may also reach the mouth by 

 contact or by the inspired air, but it is only in relatively 

 few cases that a sufficient number of the infective agents 

 will pass in this way in a living state through the 

 stomach into the intestine. 



It is further evident that the position and other con- influence of 

 ditions of the specific points of invasion must have a 

 great influence on the danger of infection. Although invasion 

 the spores of the typhoid bacilli are probably more 



