insidious

Working or spreading harmfully in a subtle or stealthy manner.

Intended to entrap; treacherous.

Beguiling but harmful; alluring.

Lying in wait; hence, deceitful; sly; treacherous.

Designed or adapted to entrap; deceptive; insnaring: as, insidious arts.

Lying in wait; watching an opportunity to insnare or entrap; deceitful; sly; treacherous; — said of persons.

Intended to entrap; characterized by treachery and deceit.

Acting or proceeding unobserved or in a seemingly harmless manner, but slowly or eventually doing great damage.

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.

Producing harm in a stealthy, often gradual, manner.