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of this acquired immunity but none have been adopted as clear 
and definite. The severity of an attack of an infectious disease 
does not affect the degree of immunity produced ; e. g., a mild 
attack of small-pox or scarlet fever will render the patient as 
thoroughly immune as a severe attack. Indeed, a severe attack 
of the disease does not always render the patient absolutely im¬ 
mune ; for cases are on record in which certain individuals have 
been known to suffer a second attack. 
For a time it was the belief of Pasteur that in an attack of 
infectious disease the pathogenic bacteria in its multiplication 
in the body of a susceptible animal exhausted some substance 
necessary for its growth, and that this substance was never re¬ 
produced. In support of this theory an artificial culture media 
may be compared to the body. When bacteria have grown in a 
given quantity of culture media for a certain time it is observed 
that further development ceases, which may be due to one or 
both of two factors, viz., exhaustion of nutrition and excessive 
amount of toxalbumins. 
We are hardly justified in supposing that the nutrition of the 
body has been used up by the bacteria, and would, therefore, 
have to conclude that the toxalbumins present in the system 
prevent any further development of the germ. At the same 
time it is hardly reasonable to suppose that these substances 
would not be excreted from the body in course of time. It 
would be just as unreasonable to suppose that a normal sub¬ 
stance in the body which had been exhausted by the bacteria 
would not again be reproduced. 
It will be seen, then, that if immunity is due to either of the 
above theories alone that it could only be temporary. It has 
been argued by some, especially Mitchnikoff, that immunity is 
largely, if not entirely due to the action of the white blood 
corpuscles which Mitchnikoff terms phagocytes. When the 
system becomes invaded with bacteria it is known that many of 
the germs are picked up and destroyed by these corpuscles. It 
is really a battle between the lucocytes and the bacteria in which 
many on each side are slain. If examined under the micro- 
