The bite from a Brown Recluse starts as a tiny wound. Gradually, as the poison begins to work its way in the area, the flesh disintegrates and the wound grows. If you want to gross yourself out, look online at Brown Recluse Spider wounds. It can become massive and very hard to heal.
I think negative thoughts have a similar manner of destroying us. What begins as a little bit of anger or irritation or negativity, inflates the more we dwell upon it. It grows bigger and uglier, and pretty soon what started as just a thought, has become a whole pattern of thinking, that then controls our behavior in a terrible downward spiral.
That must be why the Bible says "Take every thought captive for Jesus." Every thought. All behavior begins as just an idea. It would be best if all our ideas are bathed in God's radiance rather than letting poisonous ideas fester.