A phobia can develop during childhood, adolescence or early adulthood.

They're often linked to a frightening event or stressful situation. However, it's not always clear why some phobias occur.

Specific or simple phobias

Specific or simple phobias, such as a fear of heights (acrophobia), usually develop during childhood.

Simple phobias can often be linked to an early negative childhood experience. For example, if you're trapped in a confined space when you're young, you may develop a fear of enclosed spaces (claustrophobia) when you're older.

It's also thought that phobias can sometimes be "learnt" from an early age. For example, if someone in your family has a fear of spiders (arachnophobia), you may also develop the same fear yourself.

Other factors in the family environment, such as having parents who are particularly anxious, may also affect the way you deal with anxiety later in life.

Complex phobias

It's not known what causes complex phobias, such as agoraphobia and social phobia. However, it's thought that genetics, brain chemistry and life experiences may all play a part in these types of phobias.

The physical reactions (symptoms) that a person experiences when faced with the object of their fear are real and aren't simply "in one's head".

The body reacts to the threat by releasing the hormone, adrenalin, which causes bodily symptoms, such as sweating, trembling, shortness of breath and a rapid heartbeat (tachycardia).