50 CLINICAL BACTERIOLOGY. 



In the majority of cases the protection that exists is 

 against the living causes of disease themselves, and which 

 are incapable of development within the body of the particu- 

 lar animal. Much more rarely the basis of natural immu- 

 nity consists in an insusceptibility of the organism to bacterial 

 (p. 29) or similar poisons (snake-venom, ricin, abrin). As 

 examples of this so-called natural immunity to poisons may 

 be mentioned the rather marked immunity of fowl to tetanus- 

 toxin, of rats to diphtheria-toxin, and of swine to snake- 

 venom. Also in the case of the most marked immunity to 

 poisons the importance of destruction of the microorganisms 

 responsible for the poisons is not to be underestimated. 



In contradistinction from natural immunity is acquired 

 immunity. Human beings are attacked but once by a 

 number of infectious diseases a manifestation that is most 

 marked in the case of scarlet fever, of measles, and of 

 smallpox, but which is distinctly observable also in the case 

 of cholera, typhoid fever, etc. Recovery from the disease 

 has, under these conditions, conferred immunity a state of 

 protection against the same disease. All efforts to estab- 

 lish an artificial immunity through active intervention are 

 based upon this natural process. The act through which 

 this end is attempted is designated immunization or vacci- 

 nation. The immunity effected the disease-protection is 

 also designated protective inoculation. 



The oldest method of immunization is represented by 

 vaccination for smallpox. Recovery from mild vaccine- 

 disease protects against severe variolous infection. The 

 causative agent of smallpox is as yet not known, but, from 

 all else that is known concerning immunity and immuniza- 

 tion, it may be inferred that the causative agent of cow- 

 pox is identical with that of variola, and represents an 

 attenuated form of the latter a view that has an experi- 

 mental basis, especially from the investigations of Fischer 

 (Karlsruhe). 



In experimental pathology it is now possible to immu- 

 nize animals against many, if not most, infectious diseases 

 whose causative agents are known. Progress in this direc- 

 tion dates from the inoculation against anthrax undertaken 

 by Pasteur, who attenuated anthrax-bacilli by exposure to 

 high temperatures, and in this way obtained two vaccines. 

 Vaccine I (exposure for from fifteen to twenty days at a 

 temperature of 42 or 43 C. 107.6 or 109.4 F.) pro- 



