A phobia is an anxiety disorder characterised by intense, irrational fear. A very common one is arachnophobia: fear of arachnids (usually spiders). Some self-proclaimed arachnophobes are just silly. A fear of spiders can be used to get attention. Young children are common examples of this, and you can just tell them that they're not scared or say that only silly people are scared of spiders, and their fear is cured. Others are much worse, and the fear can take over their lives and change their personalities.
Phobias, and other anxiety disorders, are medical conditions and can be treated. But for some reason, people with arachnophobia tend not to think there is anything wrong with them, and they just live with their fear. Possibly the high prevalence of arachnophobia and general acceptance of it within society is to blame, but that doesn't stop people going to the doctor for antibiotics every time they get a cold or sore throat.
But if you have a phobia, you need to get it treated. Fear is controlling, and phobias can make you avoid situations that you would otherwise enjoy or benefit from. As a sufferer, you don't always realise what you are missing, but when the phobia has been treated a whole world of possibilities suddenly becomes realised.
Fear is such an intense reaction. Telling someone to stop being silly, or that there's nothing to worry about, doesn't work because fear doesn't respond to logic (otherwise it wouldn't be irrational). There are different ways of treating phobias, and I'm not a psychologist, but I have experienced some of them. If you have a phobia, go to your GP and say that it's getting in the way and you want it gone. Hopefully they will refer you to a counsellor or psychologist.
For about ten years, I had two phobias: vomiting and crowds. The crowd phobia was cured entirely by exposure: I got a job fund raising in shopping centres and hid behind the display table until I managed to brave the crowds for longer and longer periods of time. The vomiting phobia came about because I had underlying anxieties making me feel sick all the time, and I was always worried that one day it would come to fruition. I took anti-emetics, avoided children like the plague, never drank alcohol and stayed away from people who did. It got so bad that when my friends started drinking, I would run away to a dangerous place where they would not follow (even drunk people can have a sense of self-preservation). I got some treatment, mostly cognitive behavioural therapy (CBT) (break the cycle of avoidance behaviour you got yourself into), but also trying out "tapping into the brain" which is based on neurolinguistic programming. It works for people who get travel sick and respond well to placebos (which is a lot of people who get travel sick). It didn't work for me.
CBT worked pretty well for me, but the psychologist was very wise and very Glaswegian, and he would say things that stuck with me. Quotes from Mark Twain about worrying thousands of times during your life but hardly any of the things you worry about come to pass. Reminders that, no matter how intense the nausea, it will pass. I began repeating these mantras every time I felt sick rather than taking pills. I began looking forward to the time in the future when I wouldn't feel sick. Eventually, I stopped feeling sick. For some reason, the travel sickness that I developed (after 15 years of life with the strongest stomach imagineable) stuck, but I can now be around people who are drinking and eating dodgy food without worrying too much. Which means I can attend parties (though I still don't understand them). I can share a car with someone who gets carsick without leaving and deciding to walk the remaining 250 km but don't worry, I don't mind, I like walking...
With the loss of emetophobia came the rise of another phobia: claustrophobia. Any small, enclosed space, particularly if it was warm, would make me panic. It developed one night when I was trapped inside a tent in a hot, humid place. I got flashbacks of a time when I was trapped inside a car on a hot day when I was too young to understand car locking mechanisms, and the two knotted and formed a phobia. I came to NZ with this phobia, and some other ones as well (some people get moles, I get phobias, ok?). My GP referred me to a psychologist, and she treated all the underlying things that people accumulate from when they're young due to the fact that this world is hard to live in, and people can make it even harder. I understood where the fears came from, and how they worked, and that helped me deal with them. I made an anxiety ladder, each rung slightly more frightening, and exposed myself more and more to the situations which made me nervous (ranging from telephone conversations to eating in front of people). It worked really well.
So, if you are frightened of spiders or children or revolving doors or music or art, you can probably cure it very simply by exposing yourself to it (read about spiders! They are really fascinating, like music and art!). Or, if that doesn't work, get some treatment! You may be able to get it free or subsidised, and it makes life really wonderful to have a phobia lifted.