insidious

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.

At some point in time they may become the source of an insidious cancer.

Intending to entrap; alluring but harmful.

Hansel and Gretel were lured by the witch’s insidious gingerbread house.

Gashford slid his cold insidious palm into his master's grasp, and so, hand in hand, and followed still by Barnaby and by his mother too, they mingled with the concourse.

Treacherous.

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

But with whom do you contract that alliance? With the natural enemy of France — that insidious house of Austria — which detests our country from feeling, system, and necessity.