136 HOUSEHOLD BACTERIOLOGY 



Thus was revealed the significant fact that bacteria 

 may damage the body quite as much by the poisons 

 which they elaborate as by their direct presence. 



Now came the next step in the upbuilding of this 

 remarkable series of discoveries. It was found that 

 if this beef tea in which diphtheria bacilli have grown, 

 and which contains the germ-poison, be introduced 

 into an animal, at first in very minute quantities, which 

 are gradually increased in subsequent doses, the ani- 

 mal grows more and more tolerant of the poison, until 

 at last he sustains with indifference amounts which, if 

 given at first, would have been certainly and speedily 

 fatal. 



In other words, it was found that by the use of the 

 poison alone of the diphtheria bacillus in increasing 

 doses, an animal can be rendered artificially immune 

 without having suffered from the disease diphtheria 

 at all. 



But now a most incredible thing was discovered. 

 It was found that if the blood be drawn from an ani- 

 mal thus rendered artificially immune, and allowed 

 to clot, the yellowish, watery fluid which separates 

 from the solid part, and which we call blood serum, 

 contains something which, when the serum is intro- 

 duced into the body of another animal, perfectly pro- 

 tects him, not only from the poison of the diphtheria 

 germ, but from the living germ itself ; in other words, 

 enders him, too, immune.. 



