i6 INFECTIOUS AND PARASITIC DISEASES. 



During the incubation period symptoms 

 Period of are wanting, and, as far as the person is 

 Invasion. aware, he experiences nothing to indicate 



that he is not in normal health. When, 

 however, the body awakens to the impending danger, it 

 brings its defensive forces into play in an effort to throw 

 off the infectious agents and their toxins. Chnically, 

 these reactions are represented by the first symptoms 

 of disease, and are known as the ''symptoms of onset." 

 They indicate that invasion of the body by an infectious 

 agent has taken place. It is for this reason that this 

 period is spoken of as the period of invasion. 



The period of invasion also differs in various diseases, 

 and in various cases of the same disease. The symp- 

 toms of invasion may appear with startling suddenness, 

 as the chill which occurs at the onset of pneumonia, or 

 the convulsion in children which initiates most of the 

 eruptive fevers; or they may develop so gradually as 



