Insidious
Etymology
From Latin “insidiosus,” cunning or treacherous, from “insidiae,” an ambush, from “insidere,” to sit in or lie in wait (“in-” plus “sedere,” to sit). The word still crouches in ambush: an insidious threat waits, hidden, and strikes only once it is too late to stop it.
adjective · /in-SID-ee-uhs/
Working harm gradually and stealthily, so the danger is not noticed until it is advanced.
What it names
A way of doing damage — slow, quiet, and disguised — that is far more dangerous than an open attack.
Example
The most insidious part of the scheme was how normal each small step looked on its own.
Related idea
Something insidious harms by stealth and gradualness; something pernicious harms by sheer destructive force. Insidious describes the method; pernicious describes the effect.