166 INFECTION AND IMMUNITY. 



of modified smallpox,, we have an ideal vaccine. 

 In a similar manner the virulence of the anthrax 

 bacillus for sheep may be lessened by passing the 

 organism through the dove. This method of de- 

 creasing, or in some cases of increasing, the viru- 

 lence of a micro-organism was referred to in Chap- 

 ter VII under "passage." 



Attenuation. ~o single method of attenuation is suitable 

 for all organisms. Pasteur found that cultures 

 of the bacillus of chicken-cholera become so weak- 

 ened when exposed to the action of light and air 

 that they may safely be used as vaccine; also that 

 the anthrax bacillus when grown at 42 C. is at- 

 tenuated and does not form spores, and conse- 

 quently become? a suitable vaccine for sheep and 

 cattle. Of no less interest to us is Pasteur's 

 method of attenuating the virus of hydrophobia 

 by desiccating the spinal cords of infected ani- 

 mals (rabbits) ; the altered virus is then suitable 

 for the immunization of individuals who have 

 been bitten by a rabid animal. 



Work of the past decade has shown that suc- 

 cessful vaccination is possible against cholera, 

 typhoid and plague by the inoculation of aviru- 

 lent cultures, or those which have been killed out- 

 right by heat. In so far as we know the immunity 

 which is caused by vaccination or protective in- 

 oculation is antibacterial, or, better, antimicrobic. 

 This point, however, is difficult to determine in 

 relation to diseases of unknown etiology, or in the 

 event that the micro-organism does not lend itself 

 to the necessary experimental manipulations 

 (smallpox, hydrophobia). It is possible that the 

 protection may be largely antitoxic in some 

 instances. In Wright's method of the therapeutic 



