If something happens insidiously, it happens in that slow, hard-to-notice way that later causes serious harm. You mostly meet this adverb with verbs like spread, grow, and damage. 'The disease spread insidiously' means it moved through the body without a big warning. It is a formal word; in casual talk people are more likely to say 'slowly, without anyone noticing.' You would not use it for a sudden, obvious blow.