200 . ELEMENTARY BIOLOGY 



diseases, the antitoxin that is formed gradually disappears, so 

 that after some time it is possible for the recovered patient to 

 have the disease again. 



Immunity may also be acquired, as we have seen (pp. 196, 

 197), by artificial means. A passive immunity, lasting a com- 

 paratively short time, is acquired by the administration of an 

 antitoxin. This is called passive because the blood does 

 nothing -to combat the disease ; it is simply protected by the 

 substance added to it. An active immunity may be acquired 

 by the use of a vaccine, which stimulates the live cells to 

 produce the substances that cause the immunity. 



231. Disease and heredity. We know that certain diseases 

 have a way of "running in families." That is, a given family 

 may show many members that have suffered from the same 

 disease, as tuberculosis. It was commonly believed until recent 

 times that tuberculosis and other diseases are inherited, in the 

 same sense as the color of the eyes or the shape of the thumb 

 is inherited. We know that this is not true. Where tuberculosis 

 runs in a family, two facts are to be distinguished : 



1. Where one member of a family has the disease, the 

 other members are more likely to be exposed to infection 

 than they would be ordinarily, and so the disease spreads in 

 that family. 



2. Where a person has tuberculosis (or any other disease), 

 the indications are that this person has not a natural immunity 

 to the disease ; in other words, that he is susceptible to it, 

 or has a " disposition " towards it. Now it is this natural sus- 

 ceptibility (or immunity) that is inherited, and not the disease. 

 No matter how much susceptibility one had to a given disease, 

 he would not contract the disease unless he was exposed to 

 the infection by the specific microbes that cause that disease. 



