insidious

adjective3

1. •
   Causing harm in a stealthy, often gradual, manner.

   • Strong and vigorous man as he looks, Livingstone has been for years the victim of a secret and insidious disease. — 1847
2. •
   Intending to entrap; alluring but harmful.

   • Hansel and Gretel were lured by the witch’s insidious gingerbread house.
3. •
   (nonstandard) Treacherous.

   • The battle was lost due to the actions of insidious defectors.