Erythema behind the ear is one of the clinical manifestations of infectious erythema. It is a mild febrile infectious disease caused by a small viral infection, mostly seen in children under 12 years of age. It is prevalent in spring. Erythema behind the ear is an autoimmune disease, usually its attacks are also related to other factors, such as the environment, infectious pathogenic factors, genetic mutations, etc., which should be given sufficient thoughtful attention, below, we will analyze the causes of erythema behind the ear in detail. It is common in seborrheic dermatitis. The causes of seborrheic dermatitis: 1, diet is too fatty and greasy, eating too much sugar, fat, drinking alcohol, over-eating spicy and greasy food, endocrine dysfunction, digestive malfunction. 2, acne. Acne vulgaris or adult acne (toxic acne) in adolescence is not promptly eradicated, or improperly treated, can cause seborrheic dermatitis. 3, improper care, abuse of skin care products hurt the skin’s own water-oil balance function, the skin water-oil metabolism is disrupted. The PH value of the healthy skin surface is about 5.2-5.5, which is not conducive to bacterial growth. Because of the chemical attack (cosmetics, skin care products), sebum secretion increase and chemical composition of the change, and thus inhibit the bacterial effect is reduced, so that the normal flora existing on the surface of the skin such as staphylococcus, Malassezia furfur and streptococcus, etc., multiply and invade the skin and cause disease. 4, metabolic disorders, genetic factors, vitamin B deficiency and physical and chemical stimulation, very often scratching or alkaline detergents, etc. can aggravate the onset of seborrheic dermatitis. 5, mental tension, overexertion, bacterial infection. On the basis of excessive seborrhea, seborrheic dermatitis can be secondary to fungal (oval furfur spores) and bacterial (Propionibacterium acnes) infections, complicating acne symptoms; it can also be secondary to allergic reactions to fungi and bacteria, and autoimmune reactions can also be secondary to the occurrence of eczema-like lesions and disseminated seborrheic dermatitis. 6, and gonadal secretion disorder, for androgen hypersecretion caused. In addition to this, some thought that there may be immune, genetic, hormonal, neurological and environmental factors related. 7, allergic body, allergic skin type. Many seborrheic dermatitis are allergies dragged out.