insidious
Definitions
Proceeding in a gradual, subtle way, but with harmful effects.
Treacherous or crafty; working or spreading in a hidden and usually injurious way.
Examples
The insidious nature of the disease meant that it went undetected for years.
He realized too late that the insidious influence of his coworker was slowly ruining his reputation.
The software contained an insidious virus that quietly corrupted all of his important files.
Synonyms
treacherous sly cunning deceitful stealthy surreptitious crafty