You spy a scatter of white flakes on your collar. Or you’ve got an itch that won’t quit, no matter how much you wash your hair. Dandruff isn’t glamorous, but it’s common: about half the world’s population has it. An itchy, flaking scalp is rarely dangerous, but it can be annoying. Here’s what you need to know about what might be causing your flakes, what you can do about it, and when it’s time to get help. What causes dandruff? Dandruff is the mildest form of a condition called seborrheic dermatitis, a type of skin inflammation that usually affects the head, face or trunk. At its worst, this condition leads to patches of red skin covered in tiny, greasy scales. However, most people with dandruff have just minor scaliness and no redness, affecting only the scalp. Experts still don’t know exactly what causes dandruff, and it’s likely there are various contributing factors. A yeast called Malassezia is a leading suspect, says James Pathoulas, an assistant professor of dermatology at the University of Minnesota medical school …