Why is it that when a person vomits, it causes others to vomit — even when they don't have an illness causing it? Is this something triggered by the brain or is it something that happens by reflex (survival instinct) for someone to vomit since, perhaps, what the first person was eating may affect the second who vomits as well?
Vomiting after seeing someone else do it was famously depicted in the Barf-o-Rama scene in the movie, Stand by Me. An iconic, comedic scene aside, seeing others vomit can be not-so-pleasant in real life. The sound, sight, and smell, and even just watching that movie scene might have made some people feel nauseous, gag, — or even barf! While there isn’t really a definite explanation on why this phenomenon occurs, there are some theories as to why this happens (stay tuned for more on these later). But, before getting into why this occurs, it might be helpful to take a closer look at the differences between nausea, gagging, and vomiting.
Though they’re related, these three sickly terms aren’t synonymous. Think of it this way: feeling queasy or gagging doesn’t necessarily mean puking will follow. Nausea is a subjective experience that involves a wide range of triggers and sometimes an urge to vomit. While nausea is a psychological reaction, vomiting refers to the physical act of throwing up. Lastly, gagging could be looked at as a psychological or physical response. For example, someone may start to gag after seeing someone vomit or as a defensive mechanism when an object, such as a toothbrush, touches a certain part in their mouth.
More to your question, why might “contagious” vomiting happen? What's the function of such a mechanism? While there isn’t a definitive answer to this curious occurrence, there are a couple hypotheses out there. First, as you suggested, scientists believe that this may be a protective response against food poisoning. Humans have been social animals throughout evolutionary history, with survival-rooted interdependence by living and working together in groups. In the case of vomiting, consider this scenario: you and one of your early-human friends are eating meat that you don't know has gone rotten. Subsequently, your pal gets sick and starts to vomit. Then, after witnessing them upchuck their meal, you throw everything back up as well. It may be that people who were easily affected by someone else’s sickness were more likely to survive because such a response would result in releasing a harmful substance more quickly. Of course, these days, it may be less adaptive for folks that are responsible for taking care of someone who is sick.
A second hypothesis involves memories shaped by the unpleasant experience of vomiting. The sight, smell, and sounds (among other things) of vomiting might bring up some not-so-fond recollections of having puked in the past, thereby leading to nausea and gagging. Though these reflexes don’t always lead to vomiting, the power of memories alone can sometimes do the trick.
So, what’s the "cure" for this? Different people have varying levels of sensitivity to triggers. Being aware of personal triggers and limiting exposure to instances where folks are blowing chow may help. For some, it's the smell that's the trigger. For others, the sight of someone losing their lunch is what does it. Still for others, simply reading this response and thinking about it could be causing their gorge to rise. In the unfortunate event that someone nearby is tossing their cookies, lying or sitting down away from this person could potentially alleviate any gastrointestinal upset. If gagging is an unfortunate resulting reflex, relaxing with music or practicing some calming breathing exercises may bring some relief.
Hopefully some of these ideas will help assuage your urge to upchuck!Alice!