A phobia can develop during childhood, adolescence or early adulthood.
They're often linked to a frightening event or stressful situation. But it's not always clear why some phobias occur.
Simple phobias
Simple phobias usually develop during childhood. For example, a fear of heights (acrophobia). Sometimes they are called specific phobias.
Simple phobias can often be linked to an early negative childhood experience. For example, you may have been trapped in a confined space when you were young. You may then develop a fear of enclosed spaces (claustrophobia) when you're older.
Phobias can sometimes be 'learnt' from an early age. If someone in your family has a fear of spiders (arachnophobia), you may develop the same fear yourself.
Other factors in the family environment may also affect the way you deal with anxiety later in life. For example, having parents who are very anxious.
Complex phobias
It is not known exactly what causes complex phobias such as agoraphobia and social phobia. Brain chemistry, life experiences and genetics may play a part.
Content supplied by the NHS and adapted for Ireland by the HSE