Seborrheic dermatitis is an idiopathic, chronic inflammatory disease that affects areas of the skin rich in sebaceous glands such as the scalp, nasolabial folds, eyebrows, ears, and presternal and intertriginous areas. It has a bimodal distribution, with peaks in infancy and adolescence / early adulthood. Although seborrheic dermatitis has been associated with HIV infection and neurologic disease, it typically presents in healthy individuals.

The yeast Malassezia has long been regarded as the central predisposing factor to disease development. Malassezia is a normal component of skin flora, but in individuals with seborrheic dermatitis, the yeast invades the stratum corneum, resulting in free fatty acid formation and the activation of the inflammatory cascade. Changes in the skin microbiome have also been implicated in pathogenesis. Additional factors related to the environment such as low temperatures and humidity have been identified as potential triggers.