GENERAL BACTERIOLOGY 
57 
others succumb to it, or permits them to recover from 
infections while others die from the same infection. 
This “something” or “vitality,” as we frequently 
speak of it, is that peculiarity in the animal body which 
is spoken of as immunity, and we may define it as a 
lack of susceptibility to bacterial disease (which pre¬ 
vents us from contracting it) and a power of resistance 
which enables us to recover from bacterial disease (if 
the latter develops). 
It is evident, therefore, that the term “ immunity” 
expresses the idea that the resistance is not absolute 
since in some people it is so strong as to prevent them 
from infections, while in others it is only strong enough 
to secure for them a recovery from them, while in 
still others, it is not even strong enough to do that, and 
such individuals die. 
Classification and Varieties of Immunity 
Since the term “immunity” itself is only relative it 
is evident that the classification of its various types 
will also he, at best, only relative, yet it is convenient 
to consider the immunity as being of several distinct 
types. 
1. Natural, inherited or congenital immunity is that 
which exists in entire races, species or individuals at 
their very birth and is just as much one of their char¬ 
acteristics as their anatomic make-up. For example, in¬ 
fluenza and leprosy never occur—either spontaneously 
or through artificial inoculation—among animals. 
Among the different animal species there are great dif¬ 
ferences in resistance; for example, rats and dogs are 
remarkably immune to anthrax and the common fowl 
can not be infected with tetanus. 
Furthermore, within one and the same species the 
