Insidious

/ɪnˈsɪdiəs/ adjective hard

Causing harm gradually and quietly, so that the damage isn't noticed until it's already serious.

Rust is an insidious problem, weakening a bridge's beams for years before a single crack becomes visible.

Memory Trick

Picture a threat that quietly 'sits' inside a word — insidious danger sits and waits before it finally strikes.