163 BACTERIA IN RELATION TO DISEASE 



natural attack of a disease or by artificial inoculation of it. The oldest 

 protective inoculation, Jenner's vaccination for small-pox (discovered 1796), 

 started from the observation that an attack of cow-pox producing only 

 slight symptoms of disease protected human beings from the dangerous 

 variola. Up to the present day we know neither the organism of vaccine 

 nor the active principle of vaccine lymph. In the case of rabies, too, for 

 which Pasteur (162) introduced a protective inoculation with weakened virus 

 (dried tissues of rabid animals), the cause of the disease is not known. For 

 these two kinds of inoculation which formed the starting-point for all 

 modern investigations on protective inoculation we are still unable to give 

 any exact explanation. 



It was Pasteur, too, who introduced the methods of protective inocu- 

 lation for anthrax, which have been of such value in France. The practice 

 has resulted in reducing mortality from anthrax in cattle from 5 per cent. 

 to 0-3 per cent., and in sheep from 10 per cent, to I per cent. (163). 



In spite of garrulous indiscretion on the part of certain people, and 

 the dishonest cupidity of others, Koch's tuberculin inoculations (134) must 

 always have the credit of being the first method in which bacterial products 

 were used, and not the bacteria themselves, as in Pasteur's anthrax inocula- 

 tions, nor an uncertain ' something,' as was the case in vaccination. Koch's 

 method formed the basis of all the innumerable investigations that have fol- 

 lowed it, including Behring's method of serum treatment, which, as we have 

 seen, is at bottom nothing but the injection of toxine-containing serum. 



To formulate a theory of immunity that shall not be lost in clouds of 

 hypothesis is at present impossible. The alexines, antitoxines, lysines, and 

 antilysines, that the wordy research of the last few years has given us, are 

 at present quite unknown (164). The inextricably complex inter-relations 

 of the antitoxic and antibacterial properties of the body-fluids have not yet 

 been cleared up, and even the effect of normal blood-serum upon bacteria 

 is but imperfectly understood. And the greatest difficulty of all is to 

 explain the long persistence of immunity after recovery from natural attacks 

 of infectious diseases. 



