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. 



