If you don’t own one, go order an old-school hot water bottle. You can get one on Amazon for ~$10 and they feel amazing when you have a fever and your feet are freezing. They are also super-easy to use: just fill it up with hot tap water and they let out a nice even heat for ~8 hours. I am just so impressed with this technology. It’s like the Apple of cozy feet.
Anyway, I recommend getting one before you get sick.