insidious

Insidious describes harm that works slowly and quietly, so you don't notice it until it's too late. The Latin root, insidiae, means an ambush. It is about the way harm gets in, not how big it is: an insidious illness, an insidious habit, an insidious idea. Loud, sudden harm is never insidious. In the animation, a snake takes its time, and the sleeping man never wakes.