in·sid·i·ous

[in-sid-ee-uh s]

–adjective
1. intended to entrap or beguile: an insidious plan.

2. stealthily treacherous or deceitful: an insidious enemy.

3. operating or proceeding in an inconspicuous or seemingly harmless way but actually with grave effect: an insidious disease.

—Synonyms
1.   corrupting. 2.   artful, cunning, wily, subtle, crafty.