512 IMMUNITY AND PROTECTIVE INOCULATION 
of the same species. If the virulence is too great or the resistance 
below the supposed normal the vaccine may produce disease in excess 
of the amount required to establish immunity and perhaps it may kill 
the animals it was intended to protect. This has happened on several 
oceasions. On the other side, if the attenuation of the virus is too 
much, or the natural resistance unusually high, there is not disease 
(reaction) enough produced to establish immunity. In such cases the 
results are negative. Where the virulent virus is used in small doses, 
accidents have happened by way of producing a fatal disease instead 
of a mild attack that was anticipated. The difficulty rests in the 
procuring of a vaccine or the quantity of a virus that possesses just 
the amount of disease-producing power that is necessary to bring 
about immunity and no more. Thisisa balance of vital forces that it 
is exceedingly difficult to strike. 
A glance at the diseases for which active immunity has been 
attained will show that they are acute toxic affections and not those in 
which the disease consists of extensive tissue destruction. Toxic 
immunity has been attained in certain diseases but a bacterial 
immunity is much more difficult to acquire. 
The dangers in vaccination, as applied especially to animals at 
large, may be summarized as follows: 
The vaccine may be too much attenuated, resulting in the failure 
to establish immunity. 
The vaccine may be too strong (virulent) so that it will produce 
more disease than is desired, possibly causing fatal results. 
The attenuated virus of which the vaccine consists may regain 
its virulence. The distribution of living pathogenic microorganisms 
among animals is of itself not to be reeommended. They may be the 
cause of subsequent outbreaks. 
In using virulent virus in non-lethal doses, the danger of producing 
fatal results because of the susceptibility of the individual treated is 
always present. 
In the simultaneous method the danger of accident resulting from 
too strong a virus, too weak a serum, or the high resistance or unusual 
susceptibility of the individual are possible conditions to be kept in 
mind. 
Prevention. The prevention of the specific diseases of animals, 
when considered from the point of view of etiology, is not so difficult 
as is often supposed. Infectious diseases are simply parasitisms. If 
the infecting organisms can be kept away from animals the diseases 
