134 
HOUSEHOLD BACTERIOLOGY 
Now we come to immunity. We have seen that, 
under the usual conditions, the body may be capable 
of disposing of bacteria or other microbes which enter 
it by means of its cells or its, fluids, so that the in¬ 
vaders can do no harm. This condition is called here¬ 
ditary immunity—an immunity which is born with us. 
There is a good deal of difference in animal species 
in this respect. For many bacteria which are deadly 
to some of the lower animals are harmless to man, and 
vice versa. So also among the lower animals them¬ 
selves some are susceptible, some not, to the same 
species of bacteria. 
But there is another phase of immunity which we 
must look at a little more closely, called acquired im¬ 
munity. It is a very old observation of the doctors, 
which has become part of the lore of the layman, that 
there are infectious diseases in which one attack, if 
recovered from, protects its victim for a longer or 
shorter period against a subsequent attack. This is 
true of smallpox, measles, scarlet fever and in less 
marked degree of typhoid fever, diphtheria and others. 
Here is a form of acquired immunity secured 
through an experience of the disease itself. In fact, re¬ 
covery from an infectious disease can take place only 
by the establishment of an immunity which did not pre¬ 
viously exist. But this acquired immunity in some in¬ 
stances suffices only for the exigencies of the hour, 
while in others it persists for some time, precluding 
fresh infection. 
