CAUSES OF DISEASE. 13 



must be regarded as a battle to the death between the 

 body, on the one hand, and an infectious agent on the 

 other; and as the symptoms of each period correspond 

 to various phases in the battle, we are able to recognize 

 which side, for the nonce, has the upper hand. 



The incubation period is the time that 

 Incubation elapses between the entrance of the disease- 

 Period. causing microbe into the body and the 



onset of the symptoms. 

 When a person takes a drug which has the property 

 of inducing sleep, e.g., opium, or a stimulant, such as 

 whisky, there is observed an appreciable period between 

 the ingestion of either, and the beginning of the phenom- 

 ena which are characteristic for each drug. If we 

 follow these phenomena, it is observed that according 

 to the quantity taken, after they first began to appear, 

 there is a gradual increase in the number and intensity 

 of the symptoms to a certain point, when they are 

 maintained for a variable period. Finally, there is a 

 gradual lessening of the influence until the drug is 

 eliminated from the body. After the elimination there 

 remain such phenomena as are the result of the removal 

 of the influence, and the re-adjustment of the economy 

 to its normal state. The phenomena just described 

 only follow in case a dose of sufficient size has been 

 ingested, the amount varying, naturally, with the 

 individual. However, when that quantity which will 

 produce symptoms has been reached, the results are 



