Skin cells that grow and die off too fast are the cause of dandruff, but
doctors do not know why this happens. One possible cause is a fat-eating fungus
called Pityrosporum ovale, which is present in most people but to excess
in dandruff sufferers. This fungus lives on the scalp of most healthy adults
without causing any problems. Sometimes its numbers grow out of control and it
feeds on the oil made by hair follicles. This causes irritation, which leads to
an increase in skin cell shedding.
Read more about dandruff.