A phobia is an intense, irrational fear of or aversion to a specific object, situation, or activity. It is classified as an anxiety disorder where the level of fear is completely out of proportion to the actual danger, causing significant distress and avoidance behaviors that disrupt daily life.
Diagnostic and Statistical Manual of Mental Disorders (DSM-5), specific phobia criteria
A marked, persistent fear that is excessive or unreasonable, triggered by the presence or anticipation of a specific object or situation. Exposure to the trigger provokes an almost immediate fear response, and the object or situation is avoided or endured with intense distress.
A phobia typically begins one of two ways: a single frightening experience, or a gradual build-up of smaller uncomfortable moments connected to the same object or situation. Either way, the subconscious mind learns to treat that one thing as a serious threat and files it away as dangerous, no matter what the conscious mind later concludes.
Once learned, the response becomes automatic. It does not wait for permission from logic. It fires the moment the trigger appears, or sometimes just at the thought of it.