system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What actually is stress and why can it make you ill?

Like what is happening to your brain and stuff?
gpt
Stress in mammals is the brain telling the body to prepare for possible danger. The body then minimize all non-critical body functions, (like reproduction, digestion, healing, immune system) in order to save energy for possible fight or flight. If you may have to run from a tiger soon, you don't want your body wasting energy producing sperm, heal that cut or anything else not directly related to running.

It was not intended for extended period of time. But our human mind can put us in stress mode for months. Then all the non-critical functions your body have been neglecting add up.