372 SCIENCE OF HOME AND COMMUNITY 



How to control diseases. The facts which we have 

 learned in this chapter show that in order to control these 

 diseases, the first step is to take such care of the sick persons 

 that they cannot give the disease to others. In some cases 

 this means that these patients must be isolated and others 

 who may have been exposed must be quarantined in order 

 to protect the health of other people. But this alone is not 

 sufficient, because people may have mild forms of diseases 

 without being seriously sick, and yet give off disease germs. 

 Further, as already explained, some people may continue to 

 give off germs for some time after they have recovered from 

 the disease. So it is necessary to guard the public routes of 

 infection by which the germs may be carried : water, milk, 

 food, and flies. The methods of keeping water, milk, and 

 food pure have been explained in a previous chapter, and 

 the methods of controlling the fly will be discussed in a later 

 chapter. 



A few facts about the methods of controlling some of the 

 more common and dangerous diseases will now be given, so 

 that each one may do his part toward helping to do away 

 with these diseases. 



Smallpox and vaccination. It is a well-known fact that 

 after a person has recovered from certain diseases, he is pro- 

 te ted to some extent from contracting those same diseases 

 again. In some cases this protection may be complete and 

 last for years; in other cases the protection may last for 

 only a short time. The principle involved in this is the one 

 applied in vaccination as a protection against smallpox. 

 This practice was discovered over a hundred years ago by 

 an Englishman, Dr. Jenner. Cows are subject to a mild 

 disease known as cowpox, which is somewhat similar to 

 smallpox. In vaccination there is injected into a person's 

 system some virus, which contains the living active principle 

 of cowpox. This is obtained from calves raised especially for 

 this purpose. If this " takes," it causes a verv mild form of 



