system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do your muscles and joints ache so bad when you get the flu?
gpt
A few reasons. Your immune system uses white blood cells to fight off infections, and to repair your muscle fibers.  When you have the flu, the white blood cells that had been repairing muscles are diverted to fighting the flu. So some muscle fibers are not being repaired as usual, so they're sore.

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More importantly, your immune system releases some chemicals and hormones to help fight the flu, but which also cause inflammation - which also causes soreness.

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Dehydration can also cause soreness, one of the many reasons to stay hydrated when you have the flu.

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You can take an anti-inflammatory like naproxen to help with the soreness, but pay attention to the dosage, they are harmful if you take too much.  The most important things are to get plenty of rest and to stay hydrated.