pacifarin

[pə-sĭfər-ĭn]
n.
  1. Any of various bacterial products that, introduced in small amounts, protect an organism from an infection or disease without killing the infectious agent.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.