insidious in-SID-ee-uhs adjective WS #25

proceeding in a gradual, subtle way, but with harmful effects; treacherous; operating or proceeding in an inconspicuous or seemingly harmless way but actually with grave effect

1. Carbon monoxide is particularly dangerous because of its insidious nature - it's colorless and odorless.
2. The dictator's insidious erosion of civil liberties happened so gradually that many didn't notice.
3. The insidious spread of misinformation through social media undermined public trust in science.
4. The insidious disease progressed slowly, showing no symptoms until it was in advanced stages.
5. His insidious influence over the team gradually turned them against their former coach.