Scientific American: Toxicity is a Disease
http://www.scientificamerican.com/article/rude-behavior-spreads-like-a-disease/
New research by Trevor Foulk, Andrew Woolum, and Amir Erez at the University of Florida takes that same first step in identifying a different kind of contagious menace: rudeness. In a series of studies, Foulk and colleagues demonstrate that being the target of rude behavior, or even simply witnessing rude behavior, induces rudeness. People exposed to rude behavior tend to have concepts associated with rudeness activated in their minds, and consequently may interpret ambiguous but benign behaviors as rude.
I'll be honest, someone acting the fool in one of my games forces me to specifically concentrate on not responding in kind. "Ignore, report and move on" is always the best advice, but it can be difficult to follow. It takes effort and willpower.
Exposure to rude behavior clearly affects our mindset and the way we respond to rudeness, but Foulk’s final study revealed an even more unpleasant side effect of the contagion: watching rude behaviors leads us to be rude to others, and those others may then be rude (or worse) to us.
This is why I approve of Riot's "hey, you seem to be having a bad day" warnings.
How do you feel when someone starts being rude and how do you handle it?