HEALTH AND DISEASE 419 



produces a temporary exhilaration which is often mis- 

 taken for some beneficial effect. 



Immunity. In. some cases 'germs may enter the body 

 of an individual and yet have no effect on him. He 

 is said to be immune to that disease. This immunity, 

 undoubtedly, depends upon the presence of substances 

 in the body which can destroy the germs. We already 

 know that certain diseases, such as mumps, smallpox, 

 and whooping cough, we do not have a second time. On 

 the other hand, some diseases recur time and again. It 

 is not definitely known just why this difference exists. 

 Evidently the antitoxin, germicide, or other substance 

 which was produced to cure a given disease remains 

 in the body for a more or less definite period of time. 

 In some diseases this is for life. 



Questions 



1. Why should we know something about hygiene ? 



2. What are the principal causes of disease? 



3. Name eight diseases caused by germs. 



4. HOAV do germs enter the body ? 



5. Explain the incubation period of a disease. 



6. What are the ways in which the body defends 

 itself against the attack of germs? 



7. How are serums obtained? 



8. Of what value are antitoxins? How are they 

 produced? 



9. Why has it been possible to reduce the death 

 rate of smallpox? 



