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, 
renders him, too, immune. 
