BACTERIA IN DISEASE. . 1 47 



The diseases of plants known to be caused by bacteria 

 are not very numerous. Among them may be mentioned 

 pear-blight, due to micrococcus amylovorus.* Among lower 

 animals bacteria very frequently produce diseases^ — for example, 

 chicken-cholera, symptomatic anthrax, erysipelas of swine, 

 tuberculosis, anthrax and glanders. 



Koch formulated certain rules which he considered must 

 be complied with in order to prove that any microorganism 

 was the cause of a particular disease: 



First. That the organism should always be found micro- 

 scopically in the bodies of animals having the disease; that 

 it should be found in that disease and no other; that it should 

 occur in such numbers and be distributed in such a manner as 

 to explain the lesions of the disease. 



Second. That the organism should be obtained from the 

 diseased animal and propagated in pure culture outside of 

 the body. 



Third. That the inoculation of these germs in pure cultures, 

 which had been freed by successive transplantations from the 

 smallest particle of matter taken from the original animal, 

 should produce the same disease in a susceptible animal. 



Fourth. That the organism should be found in the lesions 

 thus produced in the animal. 



An infectious disease is a disease which is caused by a micro- 

 organism growing in the body of the animal having the disease. 

 Such microorganisms are usually bacteria, but not always; for 

 example, malaria is produced by a minute animal organism. 



A contagious disease is one which is acquired from direct or 

 indirect contact with an individual having the disease. Most 

 contagious diseases are infectious, but infectious diseases are not 

 necessarily contagious. The words are often used very loosely, 

 and it is no longer possible or very desirable to draw the line 



* See E. Smith. Centralblatt fur Bakteriologie , etc. Zweite Abtheilung. Bd. 

 v., p. 271; Bd. VII., p. 88. 



