This is a rare and insidious disease.

In junior high and high schools, they say insidious forms of bullying are on the rise.

That's a very insidious disease. You don't notice it until it's too late.

Experience should teach us to be most on our guard to protect liberty when the government's purposes are beneficent. Men born to freedom are naturally alert to repel invasion of their liberty by evil-minded rulers. The greatest dangers to liberty lurk in insidious encroachment by men of zeal, well-meaning but without understanding.

We are working against an enemy who is very insidious in his methods.

What an insidious assumption.

It's a dangerous and insidious animal.

If only there were evil people somewhere insidiously committing evil deeds, and it were necessary only to separate them from the rest of us and destroy them. But the line dividing good and evil cuts through the heart of every human being. And who is willing to destroy a piece of his own heart?