Plainly
look up a word, understand it — no rabbit hole
insidious
adjectivein-SIH-dee-us
meaning in plain English
Quietly dangerous, it hurts you slowly without you noticing.
for example
The insidious mold grew behind the bathroom tiles, making the wood rot before anyone saw it.
Imagine a tiny crack in a pipe that you don't hear at first. Over weeks it becomes an insidious leak that can flood a house if you don't catch it early.