It isn't an imaginary threat.
It's a grossly exaggerated threat in context, there's a few principles at play
http://en.wikipedia.org/wiki/Cultivation_theory
http://en.wikipedia.org/wiki/Mean_world_syndrome
and
http://en.wikipedia.org/wiki/Fight-or-flight_response
Television or media is a distorted version of reality but the mind isn't used to evolving with it around, as such it has some known psychological effects. If the news focus's on a specific threat for a few days/weeks from something like a terrorist attack or a shooting, the effect on the brain through excessive exposure is it exaggerate people's ability to properly asses the actual degree something is threat to them.
"""The theory clearly posits that the cultivation effect occurs only after long-term, cumulative exposure to television. "[1] He claimed that because TV contains so much violence, people who spend the most time in front of the tube develop an exaggerated belief in a mean and scary world. "[3] He posited that television as a mass medium of communication had formed into a common symbolic environment that bound diverse communities together, socializing people into standardized roles and behaviors. "Today, the TV set is a key member of the household, with virtually unlimited access to every person in the family."[3] He compared the power of television to the power of religion, saying that television was to modern society what religion once was in earlier times."""
"""
"Mean world syndrome" is a term coined by George Gerbner to describe a phenomenon whereby violence-related content of mass media makes viewers believe that the world is more dangerous than it actually is.Mean world syndrome is one of the main conclusions of cultivation theory. Gerbner, a pioneer researcher on the effects of television on society, argued that people who watched a large amount of television tended to think of the world as an intimidating and unforgiving place."""
The brain has 3 basic parts, the stem/bottom of the brain is a primal lizard brain, the mammal brain is built ontop of that and the higher order functions ontop of that, specifically the pre frontal cortex, the pre frontal cortex has the ability to supress the other parts of your brain which is driven by emotions, and allows you to fight your basic impulses to have a logical interpretation of reality and thus guide your actions correctly, the problem is when you watch television it puts you in a receptive mode that disables your ability to do that.
"""Low Alpha Waves: Causes: Radiant Light
"While watching television, the brain appears to slow to a halt, registering low alpha wave readings on the EEG. This is caused by the radiant light produced by cathode ray technology [CRT, LCDs also?] within the television set [increases serotonin levels?]. Even if you're reading text on a television screen the brain registers low levels of activity. Once again, regardless of the content being presented, television essentially turns off your nervous system."""