DANGERS IN VACCINATION 559 



doses, accidents have happened bj^ 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. 

 This is a 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 several 

 disorders but a bacterial immunity is much more difficult to 



acquire. 



As vaccination rests upon the production of artificial 

 immunity, the extent to which it can be applied depends upon 

 the efficiency of methods to produce immunity in different 

 diseases. In deciding upon the action to be taken in the pres- 

 ence of an infectious disease, the selection of a vaccine should 

 be guided by the fullest knowledge possible of the nature of 

 the disease itself and the extent to which natural and acquired 

 immunity against it exists or is made possible. 



The dangers in vaccination as applied especially to animals 

 at large may be summarized as follows : 



1 . The vaccine may be too much attenuated, resulting 

 in the failure to establish immunity. 



2. The vaccine may be too strong (virulent) so that it 

 will produce more disease than is desired, possibly causing 

 fatal results. 



3. The attenuated virus of which the vaccine consists 

 many regain its virulence. The distribution of living patho- 

 genic microorganisms among animals is of itself not to be 

 recommended. They may be the starting point of subsequent 

 outbreaks. 



4. In using non-lethal doses of a virulent virus, the dan- 

 ger of producing fatal results because of the susceptiblity of 

 the individual treated is always present. 



5. In the simultaneous method the danger of accident 



