Seborrheic dermatitis is a common skin condition that causes red, itchy, and flaky patches on the scalp, face, and other body parts. It is believed to be caused by an overgrowth of a type of fungus known as Malassezia, which naturally lives on the skin. The fungus feeds on oils on the skin, which can cause inflammation and irritation. Other factors such as stress, hormones, and immune system issues may also play a role in the development of seborrheic dermatitis.