system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What causes some people to throw up or retch if they smell something disgusting like a rotting animal, or experience something very traumatic like witness someone getting badly injured or receive a phone call with highly emotional news?

This seems to happen a lot in the movies - particularly when someone is witnessing a death or an injury, or receiving a shocking phone call - does it really happen that frequently and if so, why?
gpt
Essentially you have a few different mechanics happening here. Smells can cause nausea and vomiting in individuals because the smell causes your brain to assume that you have been exposed to something that is poisonous to you. Horrible smells cause a gag type reflex which suppresses the feeling of hunger, aiding us in avoiding bacteria ridden organic material. 

In the event of a traumatic experience or stressful situations, the need to vomit is more attributed to the "fight or flight" response. The response your body has in a stressful situation causes the release of adrenaline which heightens your sense of awareness, speeds up heart rate, and also can cause temporary boosts in strength or ability. The backside of this is that spikes in this body mechanic causes vomiting because your body works better without being weighed down by a full stomach.