A phobia is an irrational and excessive fear of a specific activity, situation, or object that triggers a compelling desire to avoid that thing. If someone has a phobia, to everyone else it may seem silly, but to the person with the phobia it is a genuine fear.
It is an unconscious automatic response to stimulus, which stems from a misperception of said stimulus. Scientists say that only two fears are hereditary or naturally instinctive: the fear of falling and the fear of loud noises. Everything else is learned. The good news is, that means it can be un-learned.