558 IMMUNITY 



used with good results much later in the period of incubation. 

 It has the additional practical feature that the number of 

 injections is minimized. 



In anthrax it is reported to be giving much better results 

 than the double inoculation with a weak and stronger vaccine 

 as followed in the Pasteur method. 



^ 450. Difficulties and dangers to be considered in 

 vaccination. The results of the efforts that have been put 

 forth during the last twent)' years to obtain control over the 

 infectious diseases of animals show that with the victories 

 there have been man}' failures. Because of the few diseases 

 for which vaccines have been successful, animal owners often 

 look upon vaccination as a safe and sure means of heading off 

 all infections. Because of their strong faith in its efficiency 

 and the ease of its application, it is often chosen rather than 

 the more difficult and perhaps immediately more expensive 

 procedures of prevention or eradication. For a few diseases 

 there is a well-established natural basis for vaccination, but 

 with others such a foundation does not appear to exist. The 

 reasons for occasional failures in vaccination are not difficult 

 to find. The analysis of the principles underlying vaccination 

 shows that it means the establishing of immunity by the intro- 

 duction into the body of non-lethal doses of virulent virus or 

 the use of a virus that has been attenuated. 



It is not always eas}' or even possible to know the exact 

 degree of virulence po.ssessed by the vaccine, and again the 

 resisting forces of animals vary even in different individuals ot 

 the same species. If the virulence is too great or the resist- 

 ance below the supposed normal the vaccine may produce dis- 

 ease in excess of the amount required to establish immunity 

 and perhaps it may kill the animals it was intended to protect. 

 This is a result that has been experienced. On the other side, 

 if the attenuation of the virus is too much, or the natural resist- 

 ance unusually high, there is not disease (reaction) enough 

 produced to cause any immunity. In this case the results are 

 negative. In cases where the virulent virus is used in small 



