INFECTION AND RESISTANCE 15 



the higher forms of animal life aids in the detection and elimination 

 of infective organisms. Not only may this be accomplished by .vol- 

 untary movement, but the purposeful action of involuntary re- 

 flexes may similarly aid the host. The body possesses a variety of 

 defenses in the form of structure, secretions, chemical substances, 

 cellular activity, all of which serve to aid in its protection in connec- 

 tion with natural and acquired resistance to disease. These will be 

 discussed in the next chapter. Plants produce certain diastases, 

 aromatic products, aldehydes, and other substances which create in 

 the plant a state deleterious to germination of harmful invaders. 

 Pigments such as chlorophyl may destroy toxic substances and 

 even bacteria in a manner somewhat similar to the action of 

 bile pigment. 



The Course of Infectious Disease. The exact moment of inva- 

 sion of an infectious agent is difficult to determine, but in cases of 

 infectious disease, the time of exposure to infection can usually be 

 stated to have occurred within the limits of a few hours. Following 

 the moment of invasion there occurs a period of incubation during 

 which the host exhibits no symptom of infection. This period of 

 incubation in some diseases is extremely variable, whereas in others 

 it is relatively fixed. In diphtheria incubation may apparently vary 

 from twenty-four hours up to nine or ten days,, and certain other 

 diseases show similar variation. In scarlet fever, on the other hand, 

 the incubation period is very commonly five days, and numerous 

 other diseases show similar fixity of incubation time. Following 

 the period of incubation the less violent infectious diseases show a 

 short period of prodromal symptoms in which headache, malaise, 

 and other minor manifestations may appear. The next period, that 

 of onset of disease or so-called invasion, may be frank or insidious. 

 Lobar pneumonia may develop within a period of a few hours and 

 exemplifies frank onset. As a contrast, typhoid fever is likely to 

 occupy a week or ten days between the period of prodromal symp- 

 toms and the full development of disease, thus illustrating insidious 

 onset. That period during which the disease is at its height is called 

 the fastigium or acme. Following the fastigium comes the period 

 of decline or defervescence. This may be by crisis or lysis. Crisis 

 is seen in approximately half the cases of lobar pneumonia, in 

 which the decline occurs in a period of a few hours. Deferves- 

 cence by lysis is seen in a large number of infectious diseases and is 

 particularly well exemplified by typhoid fever in which several days, 

 a week, or more, may be consumed. Convalescence indicates that 

 period during which the symptoms of disease have practically dis- 

 appeared and the patient gradually recovers and is restored to nor- 

 mal. At any period the infection may become so overwhelming as 

 to cause the death of the individual. Chronic infectious diseases 

 exhibit no such regularity of development and decline. In con- 

 trast to the acute infections, these are not likely to be self-limited, 

 but progress until they have reached a point of such great severity, 

 or of such complete exhaustion of the host that death ensues. 



