112 BACTERIOLOGY. 
cient power to prevent infection. Through serums, 
therefore, we can immunize against an infection, and 
even stop one just commencing; but as yet we cannot 
cure an infection which is already fully developed, 
though even here there is reason to believe that we 
may possibly prevent an invasion of the general system 
from a diseased organ as by the pneumococcus from 
an infected lung in pneumonia. On the whole, the 
serums which simply inhibit the growth of bacteria 
have not given, as observed in practice, conclusive 
evidence of great value in already developed disease. 
This is partly due to the difficulty to be discussed fully 
later of determining early enough the exact nature of 
the bacteria causing the infection. 
Acquired Immunity to Poison. Although the serum 
of animals which have been infected with any one of 
many varieties of bacteria is usually both antitoxic and 
bactericidal, still one of these protective substances may 
be present almost alone; thus antitoxic substances are 
present almost exclusively in animals injected with two 
species of bacteria which produce powerful specific 
poisons—viz., the bacilli of diphtheria and tetanus. 
When the toxins of either of these are injected in 
small amounts the animals after complete recovery 
are able to bear a larger dose without deleterious 
effects, and these doses in the more suitable animals 
can be gradually increased until a thousand times a 
previously fatal dose may be administered without any 
serious results whatever. To Behring and Kitasato we 
owe the discovery that this protecting substance accu- 
mulates to such an extent in the blood that very small 
amounts of serum are sufficient to protect other animals 
from the effects of the toxin. 
