158 MANUAL OF BACTERIOLOGY. 



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. 1 Among lower 

 animals bacteria very frequently produce diseases for 

 example, chicken-cholera, symptomatic anthrax, erysipelas 

 of swine, hog-cholera, tuberculosis, anthrax and glanders. 



Koch formulated certain rules which he considered must 

 be complied with in order to p.ove 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 cul- 

 tures, 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 

 microorganism 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 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 



1 See E. Smith in Ccntralblatt f. Baktcriologic, etc., Z \\eite Abtheilung, 

 TU1. V., p. 271 ; Bd. VII., p. 88. 



