system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why aren't there any viruses out there that make our bodies feel great?
gpt
Two reasons. The first reason is that viruses use your own cells to reproduce, which eventually destroys them (so Hepatitis virus will cause jaundice, for example). The second reason is because your immune system recognizes viruses as foreing and fights against them - most of what we call flu-like symptoms (fever, muscle/joint aches, malaise...) are side effects of your immune system fighting the infection.