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Adjective: pernicious  pu(r)'ni-shus
  1. Exceedingly harmful
    "The pernicious influence of corruption in politics";
    - baneful [archaic], deadly, pestilent
     
  2. Working or spreading in a hidden and usually injurious way
    "glaucoma is a pernicious disease";
    - insidious, subtle

See also: harmful, noxious

Encyclopedia: Pernicious