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.