CHAPTER XII 



HUMAN DISEASES CAUSED BY ONE-CELLED 



ANIMALS 



Long ago when it was first discovered that various para- 

 sitic worms lived in our bodies and were the causes of pain 

 and injury and even certain diseases physicians rapidly 

 came to believe that all our ills were in some way caused by 

 such parasites, known or unknown. Later there came a 

 reaction against this belief as the search for the supposed 

 parasites causing various diseases was unsuccessful in re- 

 vealing them; but again with the later discovery, by means 

 of perfected microscopes and methods of investigation, of 

 bacterial germs in the body tissues the parasite or germ 

 theory of disease was rehabilitated, and this time to endure. 



These first known and first studied "germs" were all 

 bacteria, which are extremely small, simple, one-celled 

 plants. They are indeed probably the simplest of all living 

 plants. But with the continued study of germs and con- 

 tagious and infectious diseases it was found that certain 

 of these diseases were produced not by bacteria or bacilli 

 but by one-celled microscopic animals, organisms belonging 

 to the branch Protozoa, or simplest animals. So today 

 just as we recognize that typhoid, cholera and tuberculosis 

 are diseases caused by the presence and growth in our body 

 of bacteria, we recognize that malaria, sleeping sickness, re- 

 lapsing fever and other related diseases are caused by the 

 presence and growth of Protozoa. 



A marked difference between the bacteria-caused and the 

 Protozoa-caused diseases is the manner of the development 

 and of the inoculation of the disease germs. While bacteria 



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