BACTERIA IN DISEASE. 169 



j these diseases. Thus, it is well known that the hydrophobia 

 I virus is to be found constantly in the brain and cord of animals 

 f suffering from rabies, and yet it has been impossible to isolate 

 and cultivate the organism. Again, the leprosy bacillus is 

 I found always present in the lesions of the disease, and yet it 

 , has not yet been cultivated and reinoculated. The spirocheta 

 I pallida of syphilis has been successfully inoculated from syphi- 

 litic lesions into monkeys, but it has not yet been cultivated 

 ! outside the animal body. 



Still, although the etiology of some diseases seems firmly 



established even where all of these postulates have not been 



I fulfilled, it is nevertheless the aim in all cases to comply with 



! them as fully as the nature of the case, and the limits of 



i present knowledge and technique will permit. 



If the fact is once established in any given case that a dis- 



i ease may be communicated from a sick individual to a healthy 



one it is classed as an infectious disease, and infectious dis- 



f eases are all caused by some living parasite. Consequently: 



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



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



i Such microorganisms are usually bacteria, but not always; for 



I 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 sharply between them. Fomites are the materials on 



which the infectious material is conveyed. 



A miasmatic disease is a variety of infection in which the 

 microorganisms are not received from another case of the dis- 

 ease, but are supposed to have been derived from the external 

 world, particularly through foul air. This word is less used 

 than formerly. 



