per·ni·cious 
(pər-nĭshəs)
(pər-nĭshəs)adj.
1. Tending to cause death or serious injury; deadly: a pernicious virus.
2. Causing great harm; destructive: pernicious rumors.
[Middle English, from Old French pernicios, from Latin perniciōsus, from perniciēs, destruction : per-, per- + nex, nec-, violent death; see nek-1 in the Appendix of Indo-European roots.]
per·nicious·ly adv.
per·nicious·ness n.