Dandruff

Real dandruff, which is a fungus infection-a sort of athlete's foot of the head--can be serious as well as unsightly. It can spread (to the eyebrows, for instance) if it is not taken care of. It almost always calls for a doctor's attention if it does not immediately respond to an over-the-counter preparation.

Be aware that one of the risks of self-treatment for dandruff is the exacerbation of a dry-scalp condition from using a medicated product.
By Richard Stein.