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 liis 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 
