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.

Infantile seborrheic dermatitis usually manifests in the second week of life and lasts 4-6 months. In infants, it characteristically presents on the face, diaper area, and folds of the neck and axillae. It usually is not associated with pruritus.

Seborrheic dermatitis has a bimodal distribution, with peaks in infancy and adolescence / early adulthood. Maternal history of seborrheic dermatitis has been found to be a significant risk factor for development of seborrheic dermatitis in infancy and childhood. 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, such as increased Staphylococcus aureus colonization, have also been implicated in pathogenesis. Additional factors related to the environment, such as low temperatures and humidity, have been identified as potential triggers.