176 THE STORY OF GERM LIFE. 



nity is beyond question. Apparently all infec- 

 tious diseases from which a real recovery takes 

 place are followed by a certain amount of pro- 

 tection from a second attack ; but with some dis- 

 eases the immunity is very fleeting, while with 

 others it is more lasting. Diseases which pro- 

 duce a general infection of the whole system are, 

 as a rule, more likely to give rise to a lasting 

 immunity than those which affect only small 

 parts. Tuberculosis, which, as already noticed, 

 is commonly quite localized in the body, has lit- 

 tle power of conveying immunity, while a disease 

 like scarlet fever, which affects the whole system, 

 conveys a more lasting protection. 



Such immunity has long been known, and in 

 the earlier years was sometimes voluntarily ac- 

 quired ; even to-day we find some individuals 

 making use of the principle. It appears that a 

 mild attack of such diseases produces immunity 

 equally well with a severe attack, and acting 

 upon this fact mothers have not infrequently 

 intentionally exposed their children to certain 

 diseases at seasons when they are mild, in or- 

 der to have the disease " over with " and their 

 children protected in the future. Even the more 

 severe diseases have at times been thus vol- 

 untarily acquired. In China it has sometimes 

 been the custom thus to acquire smallpox. Such 

 methods are decidedly heroic, and of course to be 

 heartily condemned. But the principle that a 

 mild type of the disease conveys protection has 

 been made use of in a more logical and defensible 

 way. 



The first instance of this principle was in vac- 

 cination against smallpox, now practised for more 

 than a century. Cowpox is doubtless closely re- 



