It's an odd thing, sitting at your desk or going for a walk or just watching TV and having half your vision shut off. Jan Burrell says she can still see, but it's as if she placed a hand over one eye.

It signals the start of a race that's familiar to Sky Bauman and Carol Cabanillas, as well.

Sometimes Bauman is sitting in his office and feels a sharp pain over one eye. Other times, he sees faint little zigzag lines or spots in the air, "almost like the kind of interference you see on TV."

Cabanillas is a marathon runner, but when she gets a sudden sharp pain in her left temple or eye, she knows she's starting a different kind of race.

Each of them has a very few minutes to take medications or start healing rituals that will ward off a migraine. And some people who suffer the debilitating condition might consider themselves the lucky ones. They, at least sometimes, experience "auras" — visual or other signals that precede a devastating headache. On the other hand, most migraine sufferers — about 85 percent — have the "common" type, rather than the "classic" migraine.

Those strike without warning.

An estimated 30 million Americans, three-fourths of them female, suffer from migraines, according to the American Council for Headache Education. The tendency runs in families, and symptoms often begin in childhood, although a young child may have abdominal pain and nausea rather than headache, which will come when he's older.

The American Academy of Neurologists says that 3 percent of preschool children, up to 11 percent of elementary-age children and as many as 23 percent of teens have migraines.

Headache is just one symptom of migraine, though sufferers often say it's the most memorable one. In fact, intensity is one way to diagnose migraine, says Susan Baggaley, a nurse practitioner in the University of Utah Department of Neurology. "A tension headache doesn't keep you from activities. One important element clinically is the amount of lost activity, of lost time, of disability. If you can't do anything, you are probably having a migraine."

Not all experts agree on what causes migraine. The traditional view was the blood vessels tightened, narrowing the flow of blood to the brain, and that caused the auras, if there were any. When the vessels dilated again, the pain began.

More recently, researchers have begun to believe that the brain may actually be causing the problem, although it's not as well-defined. A neurologist at Albert Einstein College of Medicine in New York told FDA Consumer magazine earlier this month that changes in the brain may trigger the blood-flow changes. And brain chemicals and the channels in nerve cells that determine the movement of minerals across them may contribute. There's no question that the vessels dilate and constrict in the process. But whether that's the cause is uncertain.