adjective/pɛʁ.ni.sjø/
insidious (having a slow and discreet, but dangerous evolution); morally harmful; pernicious; literally harmful or dangerous, usually of animals or persons
“anémie pernicieuse” — pernicious anemia, or Biermer's anemia, a form of megaloblastic anemia due to vitamin B12 deficiency
Borrowed from Latin perniciōsus.