IMMUNITY 2f)5 



taxes to provide cleaner streets, public playgrounds, proper 



sewage disposal, and adequate inspection of milk, meat, 

 and water, is really an economy. For although such 

 improvements cost money, they are not so expensive as 

 epidemics of disease and the maintenance of hospitals and 

 of orphan asylums. 



195- Heredity of Disease. — The term heredity of disease 

 is one which has been misunderstood by many people. 

 By the term heredity we mean that which is handed on 

 from parents to their offspring. In the case of biological 

 diseases which are caused by some definite' plant or animal, 

 it is evident that they cannot be inherited. But when t la- 

 parents are afflicted with a biological disease, their bodii is 

 become weakened and their offspring may have a poor 

 constitution so that they are more easily affected by disease. 



196. Immunity. — Immunity is a technical term which 

 means that the body resists or is not susceptible to the 

 germs of biological diseases. Many people do not become 

 sick when there is an epidemic of typhoid fever, measles, 

 malaria, or the like. Such people are said to possess a 

 high degree of natural immunity to disease germs. People 

 usually well frequently take germ diseases when the body 

 happens to be exhausted by care or work. In such cases 

 the immunity of the body has been weakened. Many of 

 the germ diseases confer immunity against a second attack 

 of the same disease, but tins does not hold true for all 

 persons or for all germ diseases. Vaccination a gain si 

 smallpox, in the case of most persons, confers immunity 

 for about seven years. Inoculation with the typhoid 

 serum confers immunity for from two to three years. Im- 

 munity, then, is a relative term, and depends in a large 

 measure on the state of health of the individual and on 

 his power of resisting the poisonous effects of disease 

 germs. 



