Insidious /(?)/

In·sid·i·ous

Insidious

a.
  1. Lying in wait; watching an opportunity to insnare or entrap; deceitful; sly; treacherous; -- said of persons; as, the insidious foe.
  2. Intended to entrap; characterized by treachery and deceit; as, insidious arts.
    The insidious whisper of the bad angel.
  3. Acting or proceeding unobserved or in a seemingly harmless manner, but slowly or eventually doing great damage; as, an insidious disease; an insidious plot.

Phrases & Compounds
Insidious disease
a disease existing, without marked symptoms, but ready to become active upon some slight occasion; a disease not appearing to be as bad as it really is.