246 INFECTION A^D IMMUNITY 



It is a fact well known to bacteriologists that certain of the patho- 

 genic microorganisms, when passed through several individuals of 

 the same animal species, become gradually more virulent for this 

 species. In his studies on the bacillus of hog cholera, Pasteur 9 ob- 

 served that when this microorganism was passed through the bodies 

 of several rabbits it gained in virulence for rabbits, but became less 

 potent against hogs. He succeeded, subsequently, in protecting hogs 

 against fully virulent cultures by treating them with strains which 

 had been attenuated by their passage through rabbits. 



A further principle of attenuation for purposes of immunization 

 was, at about this time, contributed by Chamberland and Roux, 10 

 who reduced the virulence of anthrax cultures by growing them in 

 the presence of weak antiseptics (carbolic acid 1:600, potassium 

 bichromate 1:5,000, or sulphuric acid 1:200). Cultivated under 

 such conditions the bacilli lost their ability to form spores and became 

 entirely avirulent for sheep within ten days. A similar result was 

 later obtained by Behring 11 when attenuating B. diphtherias cultures 

 by the addition of terchlorid of iodin. 



ACTIVE IMMUNIZATION WITH SUBLETHAL DOSES OF FULLY VIRULENT 

 BACTERIA. The use of fully virulent microorganisms in minute quan- 

 tities for purposes of immunization was first suggested by Chauveau, 12 

 and is naturally inapplicable to extremely virulent organisms like B. 

 anthracis. The principle, however, is perfectly valid, and has been 

 experimentally applied by many observers, notably by Ferran 13 in 

 the case of cholera. A similar method proved of practical value in 

 the hands of Theobald Smith and Kilborne 14 in prophylaxis against 

 the protozoan disease, Texas fever. 



ACTIVE IMMUNIZATION WITH DEAD BACTERIA. Suggested by Chau- 

 veau, the method of active immunization with gradually increasing 

 doses of dead microorganisms has been successfully employed by 

 various observers, chief among whom are Pfeiffer, Brieger, Wright, 

 and Wassermann. The method is especially useful against that class 

 of bacteria in which the cell bodies (endotoxins) have been found to 



9 Pasteur, Compt. rend, de 1'acad. des sci., 1882, t. xcv. 



10 CJiamberland et Eoux, Compt. rend, de 1'acad. dcs sci., 1882, t. xcvi. 

 u Behring, Zcit. f. Hyg., xii, 1892. 



Chauveau, Compt. rend, de Pacad. des sci., 1881, t. xcii. 

 "Ferran, Compt. rend, de 1'acad. des sci., 1895, t. ci. 



14 Th. Smith and Kilborne, U. S. Dept. of Agri., Bureau of Ani. Indust., Wash., 

 1893. 



