Many people have an affiliation or experience with a phobia that sends tingles down their spine, rational or not. What distinguishes a common fear from a phobia? Psychology tells us that a phobia is the kind of fear that warrants adjectives like "irrational" and "unreasonable." Often, that makes phobias contextual—for example, a phobia of needles (trypanophobia) is reasonable if you're alone in a dark parking lot with a crazed-looking stranger wielding a dirty hypodermic, but is irrational if you are in a hospital receiving a sterile injection from a professional. It's even more irrational if it would make you reject a medical injection when it was really necessary.
It is fears like the latter that are true phobias—fears that make people feel like they need to avoid the object of their fear at all costs. Phobias can come in all shapes and sizes, from irrational fears with a rational root, like the fear of death, spiders or lightning, to the truly bizarre, like the fear of bellybuttons, kittens or sleep.