PART V. 
CHAPTER XVII. 
PARASITIC BACTERIA. 
RESISTANCE AGAINST PARASITIC BACTERIA. 
We have learned that microorganisms may be both useful and 
harmful. If they grow where they are wanted, they are useful; 
but if they grow where they are not wanted, they produce many 
undesirable effects. They spoil foods by causing their putrefaction, 
they destroy vinegar by consuming the acetic acid. In wines the 
growth of mischievous microorganisms causes a variety of bad 
results that are sometimes spoken of as "diseases of wine," and we 
also hear of "diseases of beer." But there is no good reason for the 
use of the term here any more than in speaking of the diseases of 
butter and cheese, when unusual bacteria cause them to ripen 
abnormally. 
In most of the examples thus far studied the material upon which 
the microorganisms grow has been supposed to be lifeless, the 
bacteria existing as saprophytes. There remains the study of these 
organisms when growing upon the living tissues of animals, thus 
living the life of parasites. In the latter case they may do injury to 
the animal or plant upon which they live, thus becoming pathogenic, 
or disease germs. 
HOW MICROORGANISMS PRODUCE DISEASE. 
When they multiply inside the body, microorganisms show very 
different habits. Sometimes they become distributed over the 
whole body, located at no particular spot (blood poisoning), while in 
other cases they may be definitely localized at some one place 
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