376 BIOLOGY: GENEKAL AND MEDICAL 



In reference to all these diseases we can make the 

 following statements: 



There are a few persons who, having been 

 exposed to one or the other of the infectious 

 agents, escape illness. 



There are a few persons who, having been ex- 

 posed once or even several times without 

 resulting illness, succumb upon an additional 

 exposure. 



There are a few persons who, having been 

 infected and suffered the illness, take it again 

 after the lapse of a varying length of time. 

 There are a great number who, having once 

 suffered from one of these diseases, never take 

 it again. 



These general statements show that there are differ- 

 ences in the behavior of different individuals toward 

 the infectious diseases. They also show that acquired 

 immunity is less permanent and less uniform than natural 

 immunity. Some persons acquire little immunity 

 through infection, some soon lose the immunity, some 

 retain it many years and then lose it, some never lose it. 



Experience also leads us to believe that the per- 

 manence of immunity bears some reference to the 

 severity of the disease; to have a disease badly may, 

 but does not necessarily, guarantee a more thorough 

 and more prolonged immunity than to have it very 

 lightly. 



It may be imagined that so soon as it became clear 

 that to have a contagious disease afforded immunity 

 from future attacks, sagacious individuals set about 

 devising means by which practical advantage might be 

 made of the information. Modern methods of experi- 

 ment were, however, unknown, and the only possible 

 application during many centuries was the occasional 

 exposure of healthy persons to mild cases of the infec- 

 tious diseases in the hope that they might pass through 



