12 INFECTIOUS AND PARASITIC DISEASES. 



Character- An infectious disease follows a cycle, or 

 isTics OF course, which is typical of the class '^infec- 

 Infectious tious diseases." To be sure, there are 

 Diseases. variations from the type, sometimes very 

 pronounced, yet certainly never greater than one would 

 anticipate in view of the differences exhibited by people 

 in general. Moreover, the infectious agents causing 

 them are subject to great variations in disease-producing 

 power, a fact which influences the character of the 

 infection either for better or for worse. Concerning 

 these differences in pathogenicity as exhibited by infec- 

 tious agents, we shall have much to say in a succeeding 

 chapter. In this course or cycle of an infectious 

 disease, four stages are commonly admitted: 



1. An incubation period. 



2. A period of invasion. 



3. A stationary period. 



4. A period of decline. 



These periods, or stages, of an infectious disease 

 were first created upon purely clinical grounds; but 

 they have been retained, because they are in entire 

 harmony with modern discovery and experiment, a 

 fact which adds another laurel to the fame of those dear 

 old practitioners who knew absolutely nothing of the 

 science of bacteriology. Besides, these periods as 

 named by them, remarkable to say, graphically describe 

 the progress of microbic action from the beginning of 

 a disease to its termination. An infectious disease 



