Protection Against Disease 455 



Although the unbroken skin is usually a protection against disease 

 germs, it sometimes happens that the germs causing boils and other 

 skin diseases penetrate it at the roots of the hairs and at the open- 

 ings into the oil and sweat glands. 



Defenses. The surest way to combat disease is to prevent the 

 entrance of the germs into the body. Another most important way 

 is to keep the body in the best possible condition for fighting 

 germs if they do gain entrance. This is possible only by care- 

 ful observance of the principles of hygiene. 



The best way to keep germs from entering the body through 

 the mouth is to use only sterilized food and water. Because 

 heat destroys living germs, food that has been well cooked and 

 water that has been boiled are always safe. Although infection 

 through the air is not so common as was formerly believed, 

 it is a known fact that some germs of diseases, notably 

 those of tuberculosis, are spread by the dust that floats in the 

 air. 



Nature fortunately provides defenses against disease germs. 

 There is the skin ; there is the sense of smell which gives warning 

 against the eating of decaying germ-laden food, and notifies us of 

 the presence of foul gases. The nasal passages are guarded by 

 hairs that prevent the entrance of much germ-laden dust. The 

 trachea, through which the air passes to the lungs, is provided 

 with cilia that constantly wave upward to keep back any germs. 

 Avoidance of breathing through the mouth evidently decreases 

 the chances of germ entrance by means of air. 



The strongest natural defense against germs, however, in case 

 they actually gain entrance to the tissues of the body, is the army 

 of white blood corpuscles which is ever ready in the healthy 

 body to attack and destroy these insidious foes. Furthermore, in 

 the body that has been kept in the best possible condition by proper 

 habits of life, there are substances known as antitoxins which tend 

 to produce immunity, and certain other substances which are sup- 

 posed to make the disease germs more susceptible to the attacks 

 of the white blood corpuscles. 



Immunity. Immunity means freedom from taking disease. 

 A person is said to be immune to a disease when, though exposed, 



