system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why doe we feel like we're in a "fog" when we're sick?

Currently fending off a nasty cold, and I've felt like my brain has been working at half-power the last few days. What makes us feel so out of it when we're sick?
gpt
There's not really a better answer to 'what' other than 'the immune system'. Common viruses can't cross the blood-brain barrier, so what you're feeling is not the direct effect of the sickness. In fact, most of the symptoms - runny nose, sneezing, coughing - are really the effects of your immune system fighting off the virus, rather than the doing of the virus itself.

The reason for the general malaise that accompanies the immune response is most likely your immune system telling you "Get out of the way and let me handle this." Even if you could easily get on with your day in spite of the physical symptoms, the 'fog' motivates you to take it slow and rest so that you don't sabotage your own recovery. It's a crude measure, but we wouldn't have it if it didn't work.