herd immunity

[SOMERS at PICARD.EVMS.EDU]

The recent query about herd immunity begs the definition or explanation.
The term derives from the concept that a nonimmune individual living in
a community that has been well immunized against the organism  and who 
remains in that community (thus limiting chances for outside exposure) is
protected by the immunity of the "herd".  The notion is that immunization 
need not be universal for protection of the community or herd.  The 
unimmunized or nonexposed individual is protection by herd immunity

KDS