The shedding of skin cells from the scalp is a normal physiological process, just as it is in other parts of the body. In dandruff though, the cells that are sloughed away are more visible.
Seborrheic dermatitis occurs in parts of the body where many oil glands (sebaceous glands) are present. It appears to be due to a fungal infection with the yeast Malassezia furfur and tends to be worse in patients with serious health problems (such as HIV and Parkinson’s disease) and when sebum production is increased.
Seborrheic dermatitis may also be exacerbated during times of stress, and during the winter months. Hereditary factors may also be involved.