Insidious (adjective)

1. Working or spreading harmfully in a gradual or subtle way.
2. Treacherous and cunning.

Origin:

Late 16th century: from latin insidiosus, from insidiae "ambush, treachery".

Examples:

1. The disease had an insidious effect on her health.
2. The enemy's plan was insidious and difficult to detect.
3. The thief's behavior was insidious, making it difficult to catch him.
4. The changes were so insidious that no one noticed.
5. Her insidious remarks made the situation worse.