system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does the body "decide" if it needs to have a fever in order to fight an infection?

Some infections cause a fever while others don't. why? 

I'v had flues that made me feel almost dead, but no fever came. And I'v had colds/throat infections that were merely uncomfortable, but I did get a fever. Whats the difference?
gpt
The message to have a fever is released by some cells in the immune system when they see a bug. These messages are in the form of molecules called cytokines. 

There are many different types of cells in the immune system, and they all communicate differently. There are a lot of cytokine molecule types, so each player can communicate different things.

If an immune cell that likes to secrete "fever" cytokines finds a bug, it might send a little bit of signal. If it finds a lot of bugs and gets really mad, it might crank the fever signal way up.

Other immune cells might recognize a bug and release little to no "fever signal." This way, the immune system can respond proportionately to the threat.

Edit: It's worth nothing that inflammation helps the immune response - more immune cells show up from the blood to the battle because the blood vessels become "leakier" - leading to inflammation and swelling. However, this inflammation often causes problems when it gets out of control. You can even die from becoming "too inflamed" - it's called septic shock.