It can be an unanswered question; something left unsaid. Soft at first, it's crawling in your head. Like an Earworm tunnels through the dirt. It is a hug that starts so comfortably, but quickly begins to hurt.
It gets in through the ear, and your head becomes its home. Comfortable at first, inseparable before you know it. Then the scratching starts; soft at first you hardly notice. Then it tears into your skin; burrowing deep! It feeds until it pops.