You are probably aware of the concept of second-hand smoke, which increases the risk of disease and death. You should probably be aware of another deadly scourge: second-hand stress.
Natural selection has wired us to sense the stress of others and make it our own. If you are a gazelle and you don’t freak out when others around you do, then you might be the one about to be consumed by a predator you haven’t seen yet. Animals detect the stress of others through various sensory signals such as alarm calls, olfactory cues, or visual behaviors. Plants detect distress signals of others in the form of ethylene gas that activates their own stress response (fittingly, second-hand smoke contains ethylene). The ability to detect second-hand stress is a survival instinct that can promote evolutionary fitness.