IMMUNITY, IMMUNIZATION, AND CURE. 51 



tected against vaccine II (exposure for from ten to twelve 

 days at 42 or 43 C. 107.6 or 109.4 F.), and prelim- 

 inary treatment with the latter afforded protection against 

 virulent anthrax. The process here is quite analogous 

 to that of vaccination for smallpox. Vaccine I induces the 

 mildest degree of anthrax, recovery from which confers 

 upon the organism the ability to withstand the moderate 

 infection with vaccine II, and this in turn confers protec- 

 tion against the severest form against true anthrax. The 

 important and novel feature in this method of immunization 

 consists in the application of heat to attenuate the causative 

 agent of the disease. That which is effected by passage 

 through the body of the cow in the case of cowpox-inocu- 

 lation, the reduction of the virulence of the disease-virus, is 

 effected similarly by heat. 



As the possibility of immunization through heated bac- 

 terial cultures has been demonstrated by numerous later 

 experiments for various other infections, there can no longer 

 be any doubt as to the general applicability and the legiti- 

 macy of this procedure. The rule in question may be thus 

 expressed : Above the temperature-optimum that is, that 

 temperature at which a bacterium thrives best and is most 

 virulent between this optimum and that temperature that 

 causes death of the bacterium, there is for every variety of 

 bacteria a temperature at which, though it still lives, it 

 loses in virulence. This attenuation-temperature is for 

 most bacteria between 40 C. (104 F.) and 70 C. (158 

 F.). The higher the temperature selected, the closer to the 

 fatal temperature, the more rapidly does the attenuation 

 take place. The property thus newly acquired is, however, 

 generally not constant, the bacteria soon returning in suc- 

 ceeding generations to their original virulence. True vac- 

 cines that is, varieties of bacteria that remain attenuated 

 constantly in all successive daughter-cultures are obtained 

 only by means of the lowest possible temperature. Thus, 

 anthrax-bacilli are materially attenuated on exposure to a 

 temperature of 55 C. (131 F.) in ten minutes, and at a 

 temperature of 42.6 C. (108.7 F.) in not less than from 

 fourteen to twenty days ; but the latter only are available 

 as vaccines. When, however, all that is desired is the pro- 

 duction from a bouillon-culture of an immunizing fluid for 

 temporary use, the attenuation-temperature is made as high 

 as possible ; inasmuch as the cultivation of true vaccines in 



