18 INFECTION AND IMMUNITY. 



that, although the spirilla which cause the various 

 relapsing fevers are very similar, they can be dif- 

 ferentiated by means of immunity reactions, such 

 as the agglutinating or protective powers of im- 

 mune serums. 



A more detailed discussion of this aspect of 



general bacteriology would carry us too far afield. 



infections" Confusion sometimes arises concerning the sig- 



niid Coii- . . , ' . g 



nificance of the words infectious and contagious 

 and other words having similar roots. This con- 

 fusion is due in large part to the fact that they 

 have grown into a usage varying somewhat from 

 that which originally adhered to them, and the 

 dictionaries, even those which are medical in char- 

 acter, have hardly kept pace with the transition. 



The significance of infectious is indicated in the 

 definition of an infectious disease as given above. 

 The word contagious, on the other hand, relates 

 to a method by which some of the infectious dis- 

 eases are transmitted from an infected person or 

 animal to the healthy, namely, contact, direct or 

 indirect. Not all infections are transmitted by 

 contact, however, hence we may divide them into 

 those w r hich are contagious and those which are 

 not. Communicable is often used as synonymous 

 with contagious. 



Contagiousness is all the more striking in the 

 case of acute infections which develop rapidly and 

 soon after exposure. On the other hand, it is not 

 so striking in a disease such as pulmonary tuber- 

 culosis, which develops slowly and perhaps only 

 after repeated exposures. 



The following rather general arrangement of 

 the infectious diseases into groups according to 

 the methods, and at the same time facility, of 



