any preparation used to render an organism immune to some
   disease, by inducing or increasing the natural immunity mechanisms.
   Prior to 1995, such preparations usually contained killed organisms of
   the type for which immunity was desired, and sometimes used live
   organisms having attenuated virulence. since that date, preparations
   containing only specific antigenic portions of the pathogenic organism
   are also used, some of which are prepared by genetic engineering
   techniques.