system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are you not hungry when you're sick?

I'm almost certain this has been asked before but I couldn't find it no matter how I searched, so I apologize for that. 

Why do you stop feeling hungry when you're sick, with the flu for example? Shouldn't you want more food than usual since your body is fighting off an illness?
gpt
When you get sick, your body is trying to fight off whatever made you sick in the first place. The fighter cells need energy, so they fire off cytokines, signals that tell other cells in the immune system what to do. This causes you to be tired and want to rest, and rest gives the fighter cells more energy. The tiredness is what causes you to not want to eat as much.