system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do so many different illnesses display "flu-like" symptoms, rather than having unique symptoms, or otherwise different combinations of symptoms?
gpt
Because “flu-like” symptoms are simply indicators that your immune system is working to combat an intruder.  It is the biological version of a “Busy” light.

You might get other specific symptoms, but you’ll also get the non-specific response of fever, body aches and general fatigue.