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First, that a microscopic organism of a particular type should 

 be found in great abundance in the blood and the tissue of die 

 sick animal; second, that a pure culture should be made of 

 the suspected organism; third, that this pure culture, when 

 introduced into the body of another animal, should produce 



Fig. 93. — Robert Koch, 1843-1910. 



the disease; and, fourth, that in the blood and tissues of that 

 animal there should be found quantities of the particular 

 organism that is suspected of producing the disease. In the 

 case of some diseases this entire chain of evidence has been 

 established; but in others, such as cholera and typhoid fever, 

 the last steps have not been completed, for the reason that the 



