LECTURE XVIII. 

 CAUSES AND PREVENTION OF DISEASE. 



CONTAGITJM. 



Definition. — Living germs which constitute the specific and 

 primary cause of certain diseases and which may be transmitted 

 from one animal to another. An outbreak of disease may be 

 enzootic (local), or epizootic (widely spread). 



Description. — Bacteria are vegetable microorganisms. Each 

 individual consists of a single cell. They are of almost infinite 

 number of species and varieties, and are present in the atmosphere 

 in particles of room dust ; in drinking water ; in the soil and in 

 fact, almost everywhere in great abundance. 



Size. — They are extremely small, requiring the highest pow- 

 ers of the microscope to make them distinctly visible. It is esti- 

 mated that millions may live comfortably in a single drop of fluid. 

 An average bacillus is from 1-25000 to 2-25000 of an inch long. 



Motion. — Some of these little plants have positive motility, 

 others have no true motility ; many of them have a peculiar dan- 

 cing motion ( Brownian motion ) . 



Nutrition. — Bacteria use for their food, the chemical ele- 

 ments, oxygen, nitrogen, carbon, hydrogen, phosphorous, and 

 sulphur. Like higher beings, they vary in their likes and dislikes. 

 What is wholesome for one may be very injurious to another. 

 Each species has its certain conditions of moisture, temperature 

 and chemical reaction which are most favorable. 



Reproduction. — These little beings reproduce by fission and 

 by spore formation. If the fission is incomplete they remain in 

 chains. The spore is the hardy resistant form and corresponds 

 imperfectly to seed formation of higher plants. It is estimated 

 that one individual may increase to 1,176,570 in 10 hours. 



Plagues in history. — Throughout all the pages of history, 

 we have records of fearful plagues among men and animals. 

 About 1500 years before Christ a great plague of murrain swept 

 through Egypt and made a great slaughter of cattle. We are 



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