36 



BACTERIOLOGY. 



Racial 

 immunity 



Acquired 

 immunity 



with their resistance. Unsanitary homes and work- 

 shops, fatigue, exposure, poor nourishment, and in- 

 juries all tend to lower the resistance to disease. The 

 excessive or continued use of alcohol is a very im- 

 portant factor in lowering 1 resistance, as is shown by 

 the frequency of infectious disease, particularly pneu- 

 monia and tuberculosis, among drinkers. Constitu- 

 tional diseases like diabetes and nephritis also> lower 

 the resistance. 



It is possible to acquire immunity. Following an 

 attack of infectious disease there commonly results an 

 immunity that protects the individual from a second 

 attack. The resistance gained in this way is spoken 

 of as acquired immunity and follows diseases such as 

 measles, mumps, scarlet fever, and typhoid fever. The 

 duration of acquired immunity varies; after scarlet 

 fever it oftentimes lasts during life, while after typhoid 

 fever it may last only a year or two. That immunity 

 could be acquired in this way was known many years 

 ago, and led to the conception of producing immunity 

 artificially without actually causing the individual to 

 pass through the dangers of disease. Although not 

 the first to attempt to produce immunity artificially, 

 the experiments of Jenner, who discovered the pro- 

 tective effects of vaccination, were the most success- 

 ful. The events leading up to Jenner's discovery are 

 interesting. In England, where smallpox had been a 

 scourge for many years, it was observed that people 

 who had been accidentally infected with cow-pox, a 

 modified form of smallpox in cattle, were not attacked 



