ACQUIRED IMMUNITY 



475 



only, while a few diseases, like anthrax and glanders, are primarily 

 diseases of certain animals but may attack man. 



Immunity may be modified by external conditions. A certain 

 amount of immunity is evidently natural to individuals, races, or 

 species, but there is much variation, as we have seen, even among 

 individuals of the same family. Resistance to disease also is 

 modified by the condition of the individual exposed. Overworked, 

 tired, and " run-down " persons are much more likely to take 

 diseases than those who are in good physical trim. Resistance to 

 disease may also be weakened by the use of drugs and alcohol 

 as shown by the susceptibility of heavy drinkers to pneumonia. 



Acquired immunity. It has been a matter of common knowledge 

 for centuries that persons who once have infectious diseases do not 

 usually have them 

 a second time. A 

 Greek historian, de- 

 scribing a visitation 

 of plague in Athens, 

 more than twenty 

 centuries ago, noted 

 that those who had 

 plague and recovered 

 were safe from it 

 thereafter. The 

 Chinese, in order to 

 make their children 

 immune to smallpox, 

 gave them the dis- 

 ease in a mild form 

 by placing in the 

 nose a little of the 

 pus from one of the 

 eruptions. It was the chance statement of a dairymaid in Eng- 

 land when she said, " I've had cowpox and can't take smallpox," 

 that led Edward Jenner to make his first experiments that have 

 resulted in almost stamping out smallpox through vaccination. 

 And so today when we think of acquired immunity obtained by 



l a. tier Service 

 The first vaccination against smallpox by Dr. Jenner. 



