Seborrhoeic dermatitis is believed to be an inflammatory reaction related to a proliferation of a normal skin inhabitant, a yeast called Malassezia (formerly known as Pityrosporum ovale). The main species found in the scalp is M. globosa. It produces toxic substances that irritate the skin. Patients with seborrhoeic dermatitis appear to have a reduced resistance to the yeast.
Seborrhoeic dermatitis is not contagious or related to diet, but it may be aggravated by illness, psychological stress, fatigue, change of season and reduced general health. Those with immunodeficiency (especially infection with HIV) and with neurological disorders such as Parkinson's disease and stroke are particularly prone to it. 
From : pharmacy-and-drugs.com , pgbeautygroomingscience.com
