Ditto to everyone in the delirium/hallucinations camp, especially with a side of some hellish fever dreams.
But also, dizziness and nausea/vomiting. When even though they're lying down, perfectly still, it feels like everything is spinning and wobbling harder than the nastiest tilt-o-whirl, and it's so suffocatingly hot on top of that and it's just too much, they know they're gonna puke soon, and they should really move so they don't make a mess or choke, but they can't because everything's spinning too fast and they can't even tell which way is up, let alone muster up the strength and coordination to go that way. And maybe they start to freak out a little as it sinks in that they no longer have any control over their own body, and that just makes it worse, panicked breathing turning their stomach even more and making them lightheaded, until they finally puke and/or pass out, unable to do anything about it except pray that when they wake back up, the world will be stationary again.
Whump Trope: Reblog with your favorite side effect of a high fever