ACQUIRED RESISTANCE 279 
eases leaves the individual with his body filled with substances 
capable of resisting 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 allin 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 the 
herds from the severe and fatal infection. But to induce a mild 
attack was not easy. It would do no good to inoculate an animal 
with a small number of the bacilli which produce the disease, for 
these, by multiplying, would soon become so numerous as to 
bring about a severe attack of the disease. It was evident that 
this end could be reached only be weakening the power of the dis- 
ease-producing bacilli. After continued experimenting he finally 
accomplished his purpose by cultivating the bacilli at a tempera- 
ture somewhat above that at which they make their best growth. 
By the use of a temperature of 108°F. he obtained cultures that 
