504 IMMUNITY AND PROTECTIVE INOCULATION 
susceptible to glanders but the ruminants are immune. Black leg, 
which is very destructive to cattle, does not attack horses, the carni- 
vora or man. There are, however, instances where a species possesses 
a general immunity against a disease, but where individuals are occa- 
sionally attacked. Thus, it is not usual to find tuberculosis in the 
carnivora, but now and then a cat or a dog is found affected wih it. 
It sometimes happens that individuals belonging to a susceptible 
species resist infection. 
It has been determined that in cases of marked natural immunity 
the resistance can be overcome and the animal infected by changing 
its normal physical condition. Thus fowls that are naturally immune 
to anthrax are said to be made susceptible to it by reducing their 
temperature by immersing them in cold water. Charrin and Roger 
found that fatigue would lessen the resistance of white rats to the same 
disease. Gibier found that frogs kept at a temperature of 37° C. were 
susceptible to anthrax. 
It has been stated that sewer and other poisonous gases predisposed 
animals to infection. Abbott concluded, after a careful experiment 
in exposing rabbits to sewer gas and gases of putrid meat, that their 
resistance to infection was not lessened. Natural immunity usually 
persists under ordinary conditions throughout life. It is much more 
permanent than acquired immunity. 
Explanation of natural immunity. There are a number of explana- 
tions of this phenomenon. In brief they deal with the supposed 
actions (1) of the cells of the body, (2) of the serums or humors 
and (3) inability of infecting agents to grow in the animal body. 
Those who seek the cause for this condition in the cells find that 
phagocytosis, so ably described and demonstrated by Metchnikoff, is 
the source of the individual defence. Those who find the cause in the 
humors of the body rely upon the germicidal action of the serum itself 
or of the substances set free from the cells that are present in the 
liquids. Metchnikoff believed that the microcytase elaborated from 
the leucocytes acts as a solvent directly upon the bacteria. The split 
protein theory explains the non-development of invading organisms 
on the ground that they cannot utilize the tissues for food and conse- 
quently they perish. 
Acquired immunity. As the term implies, this is immunity 
established in the individual after birth. It is most common in 
individuals that have survived an attack of an infectious disease. The 
