Violence is a contagious disease.
As with other contagious diseases, if people are exposed to violence, their own risk of behaving violently increases. Yet, unlike most contagious diseases, a person does not need to be very close to the violence in order to be infected with it.
One can catch it at a distance. The infecting exposure can be an exposure to violence right in front of the person or it can be an exposure to distant violence through electronic media.
From “Contagion of Violence Project“, Aggression Research Program of the University of Michigan’s Institute of Social Research