Why do people have phobias? And how can you get rid of them?

Why do some people have phobias of things while others do not? I have a terrible phobia of death (bodies, coffins, morgues, cemeteries, etc..), medical things (hospitals, doctors, needles, blood, etc..) and mirrors at night. Why do we have phobias if nothing traumatic has happened to make them come up? And if you have a phobia, how can you cure it?