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Your dandruff may be a bummer, but it's harmless to your health.

Dandruff is a "cosmetic" problem, says New York dermatologist Michele Green, MD. "It will not cause anything bad."  

No one knows precisely what causes dandruff, though a fungus called malassezia may play a role. It's normal to have malassezia on your skin. Too much of it, though, is linked to dandruff.

These are some of the common reasons your scalp can get dry and itchy:

Seborrheic dermatitis (or seborrhea): This common skin condition causes redness, itching, and flaking. It can occur on many parts of the body. When it affects the scalp, it's called dandruff. It's treatable. Most often, over-the-counter medicated shampoos will stop the flakes from falling. More stubborn cases will likely benefit from prescription-strength shampoos and steroid creams.

Scalp psoriasis can cause dandruff-like itching and flaking. Psoriasis happens when the immune system triggers too many skin cells to grow on various parts of the body. That can include your scalp. People with psoriasis may be more likely to get dandruff, but psoriasis is not dandruff.

Eczema. This skin condition can make your scalp itchy and flaky. It's most common in babies and children, but anyone can get it. The cause is unknown. Moisturizers and prescription steroid creams can help relieve the symptoms, as can avoiding harsh soaps and scratching. It also helps to avoid stress whenever possible.

Certain diseases, such as Parkinson's and HIV, have also been linked to dandruff. People with these diseases have higher rates of dandruff, says Heather Onoday, RN. She's a nurse practitioner who specializes in dermatology at Oregon Health and Science University in Portland, Ore. Having dandruff doesn't make either of those diseases more likely, though.

A poor diet also has been suggested as a culprit, although Green says there's no proven link. Jeffrey Benabio, MD, agrees.

"We may see associations but we can't say that, for example, vitamin D deficiency from lack of sunlight in winter causes dandruff," says Benabio, a dermatologist with Kaiser Permanente in San Diego. 

Eat a well-rounded diet for your overall health. A dry scalp doesn't mean you're lacking any particular nutrient, though.