Here are some synonyms for insidious:
* Clandestine
* Sly
* Treacherous
* Devious
* Underhanded
* Stealthy
Here are some examples of how "insidious" can be used:
* The effects of the drug were insidious, slowly weakening his immune system.
* The company's marketing campaign was insidious, targeting children with subliminal messages.
* The dictator's rule was insidious, gradually eroding the country's freedoms.
In essence, something insidious is like a slow poison, working its way into a system without being immediately obvious.