108 Fermentation and Kindred Phenomena, 



the organisms have used up, and during that period he will 

 suffer immunity from the same disease. This, as every one 

 knows, is very often the case. 



Protection from disease. — Another question presents itself to 

 us, and perhaps it is the most important, from a practical point 

 of view, of any we have discussed. It is as follows : — 



Granted that infectious diseases are caused by organisms, then, 

 surely as we know what is required for their nourishment, and 

 what is antagonistic to their existence, we ought to be able to 

 devise some means for preventing their ravages among men and 

 animals. An answer in the affirmative to part of this question 

 was (to a certain extent) anticipated long ago, though un- 

 consciously, by the introduction of vaccination as a preventive 

 of smallpox. Perhaps few people know how ancient this 

 operation is, for it appears to have been known to the Arabs and 

 Chinese as early as the ioth century, and was also practised in 

 India by the Brahmins, a public crier announcing that he had 

 smallpox virus to sell. It was introduced into England I may 

 say accidentally in George the First's reign. 



The principle of vaccination is to give (by inoculation) a 

 mild type of small-pox, which has the effect either of rendering 

 the individual vaccinated entirely secure from an accidental 

 attack of the malady, or in case he does take the disease to very 

 materially moderate its violence. To Pasteur belongs the 

 triumph of having proved that the vaccination method can be 

 extended to other diseases incidental not only to man but to 

 animals also. Thus with spleen fever or anthrax he prepares 

 vaccine matter by cultivating the bacillus in an artificial medium 

 for a considerable time at a high temperature. By this means 

 it becomes " attenuated." 



" At the end of a week the culture which at first killed the 

 whole of ten sheep which had been inoculated with it, now 

 only killed four or five, and in ten to twelve days it ceased to 

 kill any — merely giving them a mild form of the disease and 

 protecting them from further attack " — proved by inoculating 

 them with the virulent virus, " The vaccine thus obtained in 



