If something is slowly and secretly causing harm, it's insidious — like the rumors no one seems to listen to until suddenly someone's reputation is ruined.

Insidious is related to the Latin noun, īnsidiae meaning "ambush" which comes from the Latin verb, īnsidēre "to lie in wait for." This is very fitting as an insidious rumor or problem is one whose negative effect is not realized until the damage is already done. Similarly, an insidious disease develops internally without symptoms, so that you don't realize right away that you are sick.

Definitions of insidious
  1. adjective
    working or spreading in a hidden and usually injurious way
    “glaucoma is an insidious disease”
    synonyms: pernicious, subtle
    causing or capable of causing harm
  2. adjective
    intended to entrap
    dangerous, unsafe
    involving or causing danger or risk; liable to hurt or harm
  3. adjective
    beguiling but harmful
    insidious pleasures”
    tending to entice into a desired action or state
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