insidious

salakavala Producing harm in a stealthy, often gradual, manner.
1847, George Lippard, The Quaker City: or, The monks of Monk-Hall

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.

The nurse always must be alert to signs of slow leak or insidious infiltration.

The insidious whisper of the bad angel.

All these facts clearly appear to me now to establish that the sanctioned scheme was a part of a bigger and … more insidious scheme which was to hoodwink the creditors and to firmly establish and consolidate the position …

The atmosphere of this insidious city comes out to meet him the moment he touches the European shore; for in London he meets Maria Gostrey just over from France.

This seemed to her the worst defilement into which this insidious city had cheated her and in her agitation, she nearly ran into the latrine, …

This is the insidious way sports entrap you: you follow a player, which commits you to his team. You begin to acquire scraps of utterly useless information about teammates, managers, owners, trainers, agents, lawyers.

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

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.

‘Believe me,’ he shouted, ‘these insidious folk talk dangerous nonsense. I hear they are spouting out their ridiculous platitudes not five miles from this park in which we are standing…’

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