insidious

in|sidi|ous /ɪns'ɪdiəs/
[ADJ]
Something that is insidious is unpleasant or dangerous and develops gradually without being noticed.
The changes are insidious, and will not produce a noticeable effect for 15 to 20 years...
insidiously [ADV] usu ADV adj
Delusions are sometimes insidiously destructive.

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