228 DISEASES FROM HARMFUL ORGANISMS 



ducing organisms themselves, and while not much is 

 known about the conditions which govern these differ- 

 ences in virulence, the differences themselves are of ut- 

 most practical importance, for a period of exceptional 

 virulence is almost sure to mean an epidemic of the par- 

 ticular disease. 



Defenses of Organisms. — Every infectious disease is 

 a strife between the individual infected and the disease- 

 producing organisms by which the infection is established. 

 Since not all people that are exposed to infection take it, 

 there must be a means of holding the infection at bay. 

 Furthermore, if the invading organisms were allowed to 

 develop and multiply without restraint they would pro- 

 duce poisons in sufficient quantity to destroy their host. 

 The fact that animals recover from infection is proof 

 that they have some means of combatting the organisms 

 of disease. What these are is to be learned presently. 

 . Prevention of Infection. — Disease-producing or- 

 ganisms are very widely scattered and are quite likely to 

 be taken into the body with the air that is breathed or 

 with the food or water that enters the mouth. The pre- 

 vention of infection depends in the first place on keeping 

 organisms which enter the respiratory or alimentary 

 tracts from getting in among the body tissues where con- 

 ditions for growth are favorable. This is accomplished 

 by presenting unbroken surfaces which the organisms 

 are not able to penetrate. There is no doubt that a very 

 large part of our freedom from infection depends on intact 

 surfaces. To the organisms of disease, not only the outer 

 surface of the body, i.e., the skin, but also the mucous 

 lining of the respiratory and digestive tracts, are possible 

 areas through which entrance into the body may be 

 gained. An organism which is breathed in with the air 

 is likely to lodge upon the mucous membrane of the lungs 

 or bronchial tubes, and if it penetrates the membrane 

 itj.finds itself in the warm and moist body substance where 

 growth can proceed readily. Similarly, organisms taken 



