THEORIES OF IMMUNITY. 



character, bouillon cultures, filtered, show in the 

 filtrate very slight toxic power, and the bodies 

 of the bacteria left behind often give evidence 

 of containing much more powerful substances. 



An illustration of the conditions resulting 

 from such properties is furnished by the many 

 curious facts developed in attempting to secure 

 protection against cholera and typhoid. In 

 both cases it is possible to immunize an animal 

 by the use of small and gradually increasing 

 doses of a virulent culture of the typhoid ba- 

 cillus, or cholera spirillum, but the serum of an 

 animal so immunized is found to have no power 

 of protecting another animal against injections 

 of the toxines, but only against injections of 

 the bacterium itself — representing, of course, 

 MetchnikofF's "anti-infectious" condition. The 

 same thing is true in regard to plague. 



The two groups of diseases outlined above 



should be carefully distinguished from each 



other, for there is a marked difference in the 



practical results that can be obtained in each — 



in the degree (at present), in the method, and in 



the explanations offered, in accordance with 



u 



