Inner Dialogue

Immune system: There seems to be some foreign agent present in the body.  Too early to tell if it's a virus or some kind of bacteria, but just to be safe, let's produce a lot more mucus.

Brain: Won't that make it hard to do anything since we'll be sneezing and coughing and blowing our nose a lot?

Immune system: Probably, but we got to get this foreign agent out.  Just to be safe, let's also have a mild fever.

Brain: What percentage of calories should go to making the body feel like absolute garbage?

Immune system: I don't know, all of it?  No, wait.  Can we make it so that the body is exhausted, but still wakes up in the middle of the night from other symptoms?

Brain: I don't see why not!