destroy the vitality of the pathogenic micro-organism, and also by exposure to the action of certain chemical agents.
Pasteur at once comprehended the importance of his discovery, and inferred that what was true of one infectious germ disease was likely to be true of others. Subsequent researches by this savant and by other bacteriologists have justified this anticipation, and the demonstration has already been made for a considerable number of similar diseases—anthrax, symptomatic anthrax, rouget, etc.
In Pasteur's inoculations against anthrax, "attenuated" cultures are employed which contain the living pathogenic germ as well as the toxic products developed during its growth. Usually two inoculations are made with cultures of different degrees of attenuation—that is to say, with cultures in which the toxic products are formed in less amount than in virus of full power.
The most attenuated virus is first injected, and after some time the second vaccine, which if injected first might have caused a considerable mortality. The animal is thus protected from the pathogenic action of the most virulent cultures.
Now, it has been shown by recent experiments that a similar immunity may result from the injection into a susceptible animal of the toxic products contained in a virulent culture, independently of the living bacteria to which they owe their origin. The first satisfactory experimental evidence of this important fact was obtained by Salmon and Smith in 1886, who succeeded in making pigeons immune from the pathogenic effects of cultures of the bacillus of hog cholera by inoculating them with sterilized cultures of this bacillus. In 1888 Roux reported similar results obtained by injecting into susceptible animals sterilized cultures of the anthrax bacillus. Behring and Kitasato have quite recently reported their success in establishing immunity against virulent cultures of the bacillus of tetanus and the diphtheria bacillus by inoculating susceptible animals with filtered, germfree cultures of these pathogenic bacteria.
In Pasteur's inoculations against hydrophobia, made subsequently to infection by the bite of a rabid animal, an attenuated virus is introduced subcutaneously in considerable quantity by daily injections, and immunity is established during the interval, the so-called period of incubation, which usually occurs between the date of infection and the development of the disease. That the immunity in this case also depends upon the introduction of a chemical substance present in the desiccated spinal cord of rabbits which have succumbed to rabies, which is used in these inoculations, is extremely probable. But, as the germ of rabies has not been isolated or cultivated artificially, this has not yet been demonstrated.