For 50 million Americans, allergies—often to things like pet hair, pollen, or nuts—can be simply irksome or even life threatening. Very few, though, have a mysterious allergy to vibrations, called vibratory urticarial. Running, jackhammers, lawn mowers, even bumpy bus rides can cause a person to break out in hives, develop a rash or a headache, or feel fatigued. While the allergic reaction is pretty mild, the root cause of the allergy puzzled scientists. Now a team of researchers from the National Institutes of Health (NIH) has figured out that a genetic mutation causes this rare allergy, according to a study published yesterday in the New England Journal of Medicine.