INFECTIOUS DISEASES 417 



or large tonsils are likely to take diseases easily, for disease germs grow 

 readily in these tissues (p. 193). Our powers of resisting infectious 

 diseases are lessened by intemperance, overwork, improper food, or by 

 anything else which weakens the body. Our resistance to diseases can 

 be increased by good food, fresh air, exercise, and by anything else 

 which promotes the strength and vigor of the body. 



722. Vaccines. The body can be made immune to many dis- 

 eases. One way of producing the immunity is by growing the disease 

 germs outside of the body and then killing the germs and injecting them 

 into the flesh. The small quantities of toxins that are used do not pro- 

 duce even a slight sickness, and yet they rouse the body to resist the 

 disease just as if the person really had it. In vaccination against small- 

 pox the living germs of cowpox are used. The vaccines against small- 

 pox, erysipelas, and a few other diseases can be bought at drug stores. 

 We can also buy the antitoxins against diphtheria, lockjaw, snake poison, 

 and a few other diseases (p. 386). 



723. How disease germs leave the body. Few disease 

 germs can penetrate a healthy skin, either to enter or to 

 leave the body. So the skin has little to do with the 

 spread of diseases, except those in which the skin itself is 

 affected. 



The most virulent forms of disease germs are given off 

 from the body in the discharges of the intestine and kid- 

 neys. In the days when all sewage was simply thrown 

 out of windows and doors every yard and street contained 

 great accumulations of filth, from which epidemics and 

 pestilences were widely- spread. The present rarity of 

 severe forms of epidemics is due largely to the cleanliness 

 of our houses and yards, and to proper sewage disposal. 

 Yet a great deal still remains to be done. Thus, in every 

 year improper sewage disposal still causes two or three 

 cases of typhoid fever among every thousand inhabitants 

 of the United States. The disposal of sewage is one of the 

 most important branches of government work (pp. 406- 

 408). 



OV. PHYSIOL. 27 



