HYGIENE OF RESPIRATION 



185 



the veins of a horse, the horse's blood will soon develop 

 the antitoxin of the disease. If the blood is drawn and 

 allowed to clot, clear serum may be separated. If this is 

 injected into a person suffering 

 from diphtheria, it tends to destroy 

 the germs and stop the disease. 

 It may prevent disease in the case 

 of a person who has been exposed 

 but in whom the disease has not 

 yet developed. The antitoxin treat- 

 ment has been tried without success 

 in several other diseases. Germs 

 may be transmitted by drinking- 

 water, in the food, Or through the FIG. 158. How to have a Dry 



air. Cellar - 



320. Vaccination. In 1776 E. ' dam ^ ro < 



Jenner of Berkeley, England, learned that the milkmaids 

 of the shire considered accidental cowpox, caught while 

 milking, a sure preventive of smallpox. Twenty years 

 later he began to vaccinate with cowpox 

 material to prevent smallpox. Vaccina- 

 tion protects only for a few years. If 

 exposed to smallpox, one should be vac- 

 cinated, unless he has been vaccinated 

 within the last few years. Arm to arm 

 FIG. 159. A " Trap " vaccination is dangerous. Only fresh virus 

 of Water to Prevent f rom healthy cows should be used. If the 



Gases from coming ,, , ,. 



into Houses through arm becomes very much swollen and dis- 

 Drainage and a bl e d, it is a sign that the blood has been 



Sewer Pipes. 



poisoned by the use of impure material. 



321. Why a Person can have Some Diseases only once. 



Why does not one have measles, smallpox, and some other 

 germ diseases a second time ? It is believed to be on the 

 same principle that vaccination prevents smallpox the first 

 time. After once having a disease, the antitoxin of that 



