PASTEUR, KOCH, AND OTHERS oot 
First, that a microscopic organism of a particular type should 
be found in great abundance in the blood and the tissue of the 
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 
Fic. 93.—Rosert Kocu, Born 1843. 
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 
