Phobia

A phobia is a mental illness where an individual has a severe anxiety and stress response to normal stimuli. Some phobias are merely an annoyance, but many make it nearly impossible for an individual to live a normal life.
Fear is a normal human emotion and all normal persons experience it (in fact, failure to feel fear is a rare genetic condition). Most people feel fear in common situations, such as when confronted with great heights, spiders, or snakes. However, most people can eventually control their emotional reaction to such stimuli.
However, phobics start to experience extreme anxiety even when getting anywhere close to the situation which sets off their phobia. For example, most people show no fear response to spiders that are clearly locked in a cage. However, a phobic will often not even enter a room containing a caged spider. 
A phobic reaction can be quite severe, and can be set off by even a minor exposure. Heart rate and the production of adrenaline start to increase. The respiration rate quickens. Panic often starts to set in, but unlike a panic attack, the affected individual is still in control of their muscles. The usual reaction of phobics is to do everything possible to remove themselves from the situation. 
Phobias are named with the Greek word for the item in question. Common phobias are closed in spaces (claustrophobia), the outdoors (agoraphobia), water, elevators, insects, and certain types of animals such as cats and dogs. However, there are dozens of named phobias, including such obscure and rare ones as triskadekaphobia (the number 13). 
It is unknown what causes phobias. Previous trauma often underlies phobias, but does not explain them in all patients. It also fails to explain why certain phobias of harmless things are very common (such as cats), while phobias of truly dangerous things (like automobiles) are rather rare)
In The Itch, the team treats a man with agoraphobia, fear of open spaces. Many agoraphobics feel anxiety in any type of social situation. The reaction of the patient to being removed from his home is not atypical of a phobic reaction.