Ordinary dandruff is caused by excessive drying of the skin and overactivity of the oil glands.
Climatic and genetic factors seem to be the biggest reasons it plagues some people and not others. It's not the result of poor grooming habits or a dirty brush or comb. And it's not difficult to treat, using special shampoos for a short time. First, it's important to make sure that the problem isn't an underlying scalp disease. It might be worth a trip to a dermatologist just to make sure the problem is not a symptom of a skin infection.