Parasite that takes over your body but not your brain. Most of the time, you can still go about your life; maybe once or twice a day, there's just this very brief split-second disjoint between what you try to do and what it's actually making you do. Are you ever really in control, or is it just "making" you do exactly the same things you would normally with only minor hiccups?
Your heart doesn't beat faster whenever you notice that; your endocrine system doesn't release cortisol or adrenaline. You might think fear/worry/uncertainty about the situation, but you don't feel it the way you do when a truck honks at you while you're crossing the street. You can't communicate with it in any way; it doesn't seem to be sentient, it just does things, possibly based on your own thoughts and instincts. An infected friend who's plural says it's nothing like a headmate, it's just ... controlling you.
And y'know, every so often it makes you do other things, of course.
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