PARASITES 421 



term that it is named for PASTEUR (see Art. 451). Pasteur 

 did not originate the process, but he perfected it and applied 

 it in several practical ways and succeeded in inducing people 

 to make use of it. 



484. Pasteurization and the Spread of Disease. This delay 

 in the souring of milk is by no means the most important 

 result of the practice of pasteurizing milk. A far more im- 

 portant result lies in the fact that disease-causing bacteria 

 which the milk may chance to contain are effectively killed 

 by the process. Milk is an ideal food for most bacteria and, 

 for this reason, it often becomes a means for the spread of 

 bacterial diseases, such as typhoid fever, tuberculosis, and 

 diphtheria. None of the bacteria which cause the more com- 

 mon contagious diseases are known to form spores and, there- 

 fore, any such bacteria are effectively killed by pasteurization. 



II. PARASITES 



485. Parasites and Their Hosts. You will recall the state- 

 ment made in Art. 433 that organisms which live and find 

 their food within the bodies of other organisms are known 

 as PARASITES. The organism within whose body a parasite 

 lives is known as the HOST. Parasites often produce certain 

 poisons, or TOXINS, in the body of the host. These poisons 

 attack certain parts of the body of the host and thus the para- 

 sites become the cause of what we commonly call diseases. 

 The larger animals are affected by many kinds of animal 

 parasites, such as tapeworms, trachina, and the malarial 

 parasite, but most of the common diseases are caused by plant 

 parasites and the bacteria are by far the worst offenders in 

 this way. A good many different kinds of bacteria, how- 

 ever, are known to live and thrive within the mouths and 

 alimentary tracts of man and of other animals which are not 

 known to cause any harm to their hosts. Indeed, it is thought 

 by some bacteriologists that some of these bacteria are of 

 benefit to their hosts in some unknown way. 



