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 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 
