CAUSES OF DISEASE. 37 



the words were synonymous, we would scarcely describe 

 a disease, such as diphtheria, as being both infectious 

 and contagious. A disease may be contagious, or it 

 may be infectious; or it may be both contagious and 

 infectious. Malaria is an infectious disease, but not 

 contagious; scabies (itch) is contagious but not infec- 

 tious; diphtheria is both contagious and infectious. 

 Diseases exhibit at times, however, marked differences 

 in regard to these characteristics. Sometimes an infec- 

 tious disease not ordinarily contagious may, in some 

 manner as yet not understood, become contagious. 

 This happens in pneumonia, which under such cir- 

 cumstances partakes of the characteristics of an epi- 

 demic disease. 



Of late years it has seemed best to some 



authors, on account of the indifferent 



CABLE. ' . , . , , , . . . , 



manner m which the words mfectious and 



contagious have been used in describing a disease, to 

 omit them entirely, and in their stead to speak of these 

 diseases as coimnimicahle. This seems an excellent 

 innovation because only diseases caused by living things 

 are, strictly speaking, communicable. An objection 

 might be made to the term because it would include 

 parasitic affections, but this cannot for long have any 

 real force ; it is difficult at best to draw a nice distinction 

 between many diseases due to parasites and the infec- 

 tious agents, and we might just as well for good have 

 a word adaptable and usable for both. 



