258 PARASITIC BACTERIA. 



acquired immunity. It has long been known that if a person has one 

 attack of certain diseases and recovers, he is, for a time at least, 

 protected from a second attack of the same disease. It is not common, 

 for example, to have scarlet fever twice, and the same is true of a 

 number of other diseases. This acquired immunity is, however, 

 quite variable. In some cases it is almost a perfect protection for 

 life, or at least for many years. With other diseases it is weaker, 

 affording only a partial protection and lasting only a few months or 

 perhaps only a few weeks. The question of what causes this ac- 

 quired immunity is closely akin to what causes race or individual 

 resistance. Doubtless the two are closely related and are probably 

 attributable to the same general cause. For our purpose it is only 

 necessary to know that recovery from one of these diseases leaves 

 the individual with his body filled with substances capable of re- 

 sisting the kind of bacteria that produced the disease. As long 

 as these resisting substances are present the individual will have 

 immunity. 



It is somewhat surprising that recovery from a mild attack of one 

 of these diseases gives as much immunity as recovery from a severe 

 attack. Hence, with this principle in mind, the question has arisen 

 whether it may not be possible to give an individual a mild case of 

 some of the more dangerous diseases in order to give him power to 

 resist the more severe and perhaps fatal types. This was first done 

 in the case of smallpox, which has for a century been fought upon 

 this principle, since the vaccination pustule seems to be essentially a 

 mild type of smallpox. Hence, when a person is vaccinated, he is 

 given a mild form of smallpox, and this guards him from a more 

 severe attack. That vaccination is a protection against smallpox 

 is pretty generally admitted to-day, although some deny its power. 



But whatever be the facts in regard to smallpox, there is no 

 doubt at all in regard to the successful application of this principle 

 to other diseases. Pasteur was the first to attempt an application 

 of this principle to a disease other than smallpox. He was at the 

 time working upon a serious disease of cattle anthrax; one that 

 is practically always fatal. He argued that if he could find 

 means for producing a mild type of the disease, he might protect 



