1. proceeding in a gradual, subtle way, but with harmful effects
2. having a gradual and cumulative effect that is difficult to detect
Usage: often used in medical contexts

Examples

• The insidious effects of pollution became apparent only after decades.
• Carbon monoxide is particularly dangerous because it’s an insidious poison.
• The disease had an insidious onset, with symptoms appearing gradually.
• Social media can have an insidious influence on young people’s self-esteem.
• The insidious spread of misinformation threatens democratic institutions.
• Racism often works in insidious ways that are hard to recognize.
• The insidious nature of the addiction made it difficult to treat.