52 CLINICAL BACTERIOLOGY. 



the case of the more parasitic varieties, which spontaneously 

 lose their virulence gradually in culture, has not hitherto 

 proved successful. The introduction of a living bacterial 

 culture attenuated by means of such a degree of tempera- 

 ture confers immunity to the corresponding bacterial disease. 

 The act of immunization induces a mild disease, recovery 

 from which gives rise to the immunity. For this reason the 

 advent of immunity is, under these circumstances, never im- 

 mediate, but it takes place only after the termination of the 

 mild disease induced. According to the degree of atten- 

 uation, and the amount of bacteria introduced, as well as 

 the mode of introduction, the time that elapses before im- 

 munization is complete varies from three to fourteen days. 



The role of heat in the attenuation of cultures employed 

 for immunization may be assumed by a number of other 

 factors. Thus, immunization has been practised with cul- 

 tures through which electric currents have been passed 

 for some time ; further, with cultures that have been ex- 

 posed to sunlight ; but most commonly with cultures that 

 have been exposed to the action of chemic substances (anti- 

 septics). Of the large number of these substances and 

 methods of immunization, we shall mention only the 

 attenuation of anthrax-bacilli by the addition of potassium 

 bichromate or of carbolic acid ; further, immunization 

 to diphtheria and to tetanus by means of cultures to 

 which iodin trichlorid has been added ; and immunization 

 by means of thymus-bouillon-cultures, in which the cellular 

 substances of the thymus gland are supposed to exercise 

 the attenuating influence. 



Finally, the living organism also possesses under certain 

 circumstances the capability of diminishing the virulence 

 of microorganisms. Pasteur demonstrated this first for the 

 bacillus of swine-erysipelas, by passing this organism 

 repeatedly through the bodies of rabbits, which are but 

 slightly susceptible. According to this method, two vac- 

 cines against swine-erysipelas of different strength are 

 obtained, which are employed largely and successfully in 

 France. Pasteur also obtained a marked reduction in the 

 virulence of the virus of hydrophobia through continued 

 inoculation of monkeys. Cowpox, as has already been 

 stated, represents likewise only an attenuated modification 

 of smallpox, the attenuation being effected by the passage 

 of the disease through cattle. 



