system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is vomiting a reaction to horrifying sights?
gpt
Vagal nerve overstimulation, same as getting punched in the gut, kneed in the crotch, getting dizzy or having a high fever can do.  Basically a one-size-fits-all "bad things happening" response from the part of your nervous system that normally handles unconscious or semiconscious processes like breathing, digesting, etc.

Also why defecation is a barometer of extreme fright.