The Collaborative International Dictionary of English v.0.48:

Pestilential \Pes`ti*len"tial\, a. [Cf. F. pestilentiel.]
   1. Having the nature or qualities of a pestilence. "Sends the
      pestilential vapors." --Longfellow.
      [1913 Webster]

   2. Hence: Mischievous; noxious; pernicious; morally
      destructive.
      [1913 Webster]

            So pestilential, so infectious a thing is sin.
                                                  --Jer. Taylor.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pestilential
    adj 1: likely to spread and cause an epidemic disease; "a
           pestilential malignancy in the air"- Jonathan Swift;
           "plaguey fevers" [syn: pestilent, pestilential,
           pestiferous, plaguey]