410 GENERAL SCIENCE 



(c) How germs are resisted by the body itself. 



(d) How germs may be resisted by other means. 

 Source of Germs. Since the air is filled with germs, 



many of them disease-producing, we cannot avoid receiv- 

 ing some of them into our systems. These are transmitted 

 in food and drink and by contact with animals or sick 

 persons. 



Conditions Favorable to Growth. Bacteria thrive 

 and multiply rapidly if the surroundings into which they 

 are introduced are favorable to their growth. Moisture, 

 a moderate degree of warmth and darkness, and food 

 from animal or vegetable matter are the conditions in 

 which they thrive best. The bodies of men and animals 

 furnish all these conditions. Food and drink are also 

 favorable to germ growth. 



Resisting Power of the Body. - - The body easily re- 

 ceives the harmful germs through its natural openings, 

 lined as they are with the warm, moist, mucous membrane, 

 and through cuts and scratches. The body, however, 

 has very efficient means of protecting itself. Some of 

 the white corpuscles of the blood, called phagocytes, 

 absorb disease-causing bacteria as their food, and success- 

 ful resistance to infectious diseases is thought to depend 

 largely upon the number and activity of these phagocytes. 

 Then, too, the body produces certain other substances 

 which are germ destroyers. 



When disease germs enter the body, there are not enough 

 of them to produce the disease at once. Usually a period 

 of time varying with the kind of disease germ elapses, 

 before the symptoms of the special disease appear. This 

 time, known as the period of incubation, is occupied by 

 the germ in multiplying and producing violent poisons 

 called toxins. These poisons are absorbed by the blood 



