What is "insidious disease"

Insidious \In*sid"i*ous\, a. [L. insidiosus, fr. insidiae an ambush, fr. insidere to sit in; pref. in- + sedere to sit: cf. F. insidieux. See Sit.]

1. Lying in wait; watching an opportunity to insnare or entrap; deceitful; sly; treacherous; -- said of persons; as, the insidious foe. ``The insidious witch.''
   --Cowper.

2. Intended to entrap; characterized by treachery and deceit; as, insidious arts.

   The insidious whisper of the bad angel.
   --Hawthorne.

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.

   Insidious disease (Med.), 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.

   Syn: Crafty; wily; artful; sly; designing; guileful; circumventive; treacherous; deceitful; deceptive. -- In*sid"i*ous*ly, adv. -- In*sid"i*ous*ness, n.