CAUSE AND PREVENTION 



LECTUEE XIX 

 CONTAGIUM 



Any living virus, either plant or animal, which constitutes the 

 specific and primary cause of disease, and which may be trans- 

 mitted from one animal to another is called a contagium or an 

 infection. An outbreak of animal disease may be enzootic (lo- 

 cal), or epizootic (widely spread). 



Description. — In the lower forms of life, bacteria and pro- 

 tozoa, each individual consists of a single cell. The higher 

 forms such as molds, are larger and more complicated. Bac- 

 teria molds, etc., are present in the atmosphere on particles of 

 room dust, in drinking water, in the soil, and, in fact, almost 

 everywhere in great abundance. 



Size. — Bacteria (plants) and protozoa (animal) are extremely 

 small, requiring the highest powers of the microscope to make 

 them distinctly visible. It is estimated that millions may live 

 comfortably in a single drop of fluid. Bacilli vary from about 

 1/25000 to 2/25000 of an inch in length. Approximately six 

 red blood corpuscles in a row would fit across the cut end of a 

 fine hair, and about six ordinary bacilli in a row would reach 

 across one corpuscle. 



Motion. — Some of these forms of life have the power of self- 

 movement, while others have no true motility. 



Nutrition. — Bacteria use for their food both organic and in- 

 organic food materials of wide variety. 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 corre- 

 sponds imperfectly to seed formation of higher plants. It is 

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

 hours. 



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