14 INFECTIOUS AND PARASITIC DISEASES. 



directly dependent upon the amount taken. There 

 is no increase of the drug within the body. 



When, however, we come to study the action of an 

 infectious agent upon the body, we find the same 

 interval of time between the entrance of the infectious 

 agent and the onset of the symptoms, but there is a 

 very great difference in the events that take place to 

 produce the reactions. The amount of opium or 

 whisky ingested is fixed, so that whatever the effect, it 

 results from just that amount. When infectious agents 

 enter the body, however, they at once begin to multiply 

 in enormous numbers, giving off all the while as a 

 product of their activity most powerful poisons (toxins) . 

 When a sufficient amount of poison has been generated, 

 symptoms of the disease appear. In other words, the 

 infectious agents that gain an entrance into the body 

 have not of themselves sufficient power to produce the 

 disease, but act through their facility of increasing their 

 numbers. It is self-evident, therefore, why this period 

 is called the "incubation period," since it "corresponds 

 to the development (multiplication) of the pathogenic 

 agents." Clinically the incubation period is also 

 known as the latent period. 



The incubation period varies as to time in different 

 diseases, and in different cases of the sam.e disease. 

 It may only be a few hours, as in erysipelas, or it may 

 extend over months and years, as in hydrophobia and 

 leprosy. This period is of very great importance from 



