Source : Webster's Revised Unabridged Dictionary (1913)

Pestilential \Pes`ti*len"tial\, a. [Cf. F. pestilentiel.]
   1. Having the nature or qualities of a pestilence. ``Sends
      the pestilential vapors.'' --Longfellow.

   2. Hence: Mischievous; noxious; pernicious; morally
      destructive.

            So pestilential, so infectious a thing is sin.
                                                  --Jer. Taylor.

Source : WordNet®

pestilential
     adj : likely to spread and cause an epidemic disease; "a
           pestilential malignancy in the air"- Jonathan Swift;
           "plaguey fevers" [syn: {pestilent}, {pestiferous}, {plaguey}]