PART II 



CHAPTER XXVIII 



PARASITIC PROTOZOA CAUSING DISEASES OF MAN 

 AND DOMESTIC ANIMALS 



Parasites and Disease. More than two hundred years 

 ago it was known that small animal parasites were associated 

 with and were the cause of certain diseases, and it soon came 

 to be generally believed that all of our ills were in some way 

 caused by such parasites, known or unknown. At first only the 

 parasitic worms and other large parasites were known, but 

 later it was discovered that many of the Protozoa and minute 

 plant parasites, the bacteria, also caused many diseases. To- 

 day we know definitely that such diseases as typhoid, cholera, 

 tuberculosis and many others are caused by the presence of 

 bacteria in the body and that such maladies as malaria, 

 sleeping sickness, syphilis, and many fevers are caused by 

 Protozoa. 



Then there is a long list of other epidemic diseases, such as 

 smallpox, measles and scarlet fever, the exact cause of which 

 has not been determined. Many of these are believed to be 

 due to micro-organisms of some kind, and if so these parasites 

 will almost certainly sooner or later be found. Curiously 

 enough most of the diseases in this last class, and many of those 

 that are caused by bacteria, are contagious, while all that are 

 caused by animal parasites are, as far as is known, infectious 

 but not contagious. It is important that we keep in mind this 

 distinction. By contagious diseases are meant those which 

 are transmitted by contact with the diseased person, by touch, 

 or by the breath or the effluvia from the body, or by the use 



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