The sudden loss of a piece of hair may be related to pemphigus vulgaris, seborrheic dermatitis, fungal infection and other reasons. 1. Pemphigus vulgaris: Pemphigus vulgaris is a sudden onset of limited patchy alopecia, with clear boundaries of the alopecia area, no scarring, no inflammatory reaction and other manifestations, sudden loss of a piece of hair may be caused by pemphigus vulgaris. 2. Seborrheic dermatitis: Seborrheic dermatitis is a common chronic superficial, inflammatory skin disease, when seborrheic dermatitis occurs in the head, it may cause limited alopecia, which leads to a sudden loss of hair. 3. Fungal infection: when the patient’s scalp area is infected by fungi, the scalp area may appear erythema, pimples, blisters and dandruff and other skin lesions, the serious condition will lead to hair loss. It is recommended that patients seek timely medical examination to clarify the cause of the targeted treatment, pay attention to personal hygiene in life, keep the scalp area clean and dry, avoid violent scratching or friction, so as not to cause skin lesions or infection.