Fear is very real and is not a disease of the mind. An irrational fear, otherwise known as a phobia could be termed as a mental disorder, however fear itself is as natural as love when brought about by tings which ought to cause fear.
From an evolutionary perspective we fear things which may cause us harm in order to ensure our survival. We become aroused and then we label that emotion as fear. Emotions according to the Shachter singer theory are the excitation/arousal caused by hormones such as adrenaline and cortisol along with the cognitive label which the human ascribes to it.
We have fear so that our amygdala can send signals to the adrenal glands to release adrenalin and bring about the flight or fight response. That is it gives us remarkable bursts of energy which can allow human beings to accomplish tasks at a higher level than they would normally do so- there are some extremely interesting cases.
Most people would be scared if they came face to face with a hungry lion, or a shark. The response is innate and natural and I would argue, as would many evolutionary psychologists, that we are born with an innate fear of things which cause us harm.
I believe irrational fear, such as that expressed by phobia has been learned by the individual and can be unlearned via treatments such systematic desensitisation or flooding.
In the case of phobias and other disorders such as OCD, the patient is often made to feel fear, and then with the use o breathing exercises just allowed to feel it, to prove that it reaches a height and then fades away- however, in actual situations which should cause fear, this is not the case- you will either fight the thing causing you fear or run, it is a survival mechanism.
So the overused statement "You have nothing to fear, but fear itself" is just silly!