pernicious

adjective4

1. •
   Causing much harm in a subtle way.

   • pernicious effects
   • They warned of the pernicious effects of misinformation.
2. •
   Insidiously villainous: intending to cause harm, especially in a subtle way.
3. •
   Causing death or injury; deadly.
4. •
   (no longer used) swift; celeritous.