8 INFECTIOUS AND PARASITIC DISEASES. 



can terminate only by the destruction of one of the 

 combatants. 



In contrast to this monster evil is the lesser, 

 Parasite, the parasite. A parasite is an organism 



that lives within or upon another organism 

 called the host. The parasite's purpose is an easy 

 living at the expense of the host. It subtly recognizes 

 that it is to its interest not to inflict too great an injury. 

 If perchance it causes the death of the host, it is an 

 accident. It seldom invades the body generally. 

 From the foregoing is seen the reason for drawing a 

 distinction between parasites and infectious agents 

 based upon their mode of action and the effects they 

 produce. It should be remembered, however, that 

 parasites and infectious agents are not necessarily 

 represented by distinct organisms. Indeed, the same 

 microbe may live upon our bodies, or within its cavities, 

 at one time as a parasite, and at another time be the 

 cause of an infectious disease. The germ of pneu- 

 monia is a constant inhabitant of almost everyone's 

 mouth, leading there a harmless parasitic existence, 

 yet let the vital powers be reduced through fatigue, 

 exposure, or cold, and it becomes an infectious agent 

 through the disease (pneumonia) which it provokes. 

 The difference between a germ that is at one time a 

 parasite, and at another time an infectious agent, 

 depends in the latter case upon the power it has of 

 producing its specific poison, and also upon its chance 



