How do kids bounce back like this, and why don't adults?

I have seen this over and over again. Little kids, under the age of 9 or so, can be running a pretty high fever and feeling bleh and lethargic, laying in bed, but as soon as that fever breaks they’re back at it, climbing the walls. There’s no recovery time at all needed, apparently.
Sometimes that fever thing can go on over a couple of days. They have no symptom but the fever. If the fever is up, they’re down, if the fever is gone, they’re normal.

This morning my 6 year old grand daughter stayed with me because she was sick and couldn’t go to school. She was vomiting and had no appetite. (Don’t know if she had a fever because Mom gave her Tylenol before she brought her over.)
After she’d been here a couple of hours she had the worst bout of vomiting yet…..and suddenly, it was all over. She was healed!
I walked into the bathroom to clean up supplies, and in the few minutes I was gone, I came out and she was jumping on the couch. She was suddenly ravenous and scarfed down two cinnamon rolls.

I have seen it happen just this way, over and over and over in my 40 years of working with kids.

Why can’t / don’t adults recover instantly like that? Do we just want to prolong the feeling sorry for ourselves, or are we just slower to recover?