A bag on the back that hurts when touched may be caused by diseases such as folliculitis, boils and epidermal cysts. 1. Folliculitis: It is an inflammatory skin disease caused by the involvement of hair follicles, and the cause is mainly related to infection. Red papules appear locally, accompanied by tenderness, and are most common on the head, face, and back. 2. Boils: acute purulent inflammation of the hair follicles, mainly caused by Staphylococcus aureus infection. Patients will have localized painful nodules, red, hemispherical symptoms, and the pain is aggravated when touching. 3. Epidermoid cyst: It is a kind of cyst containing keratin inside the dermis. Usually manifested as a dome-shaped elevated skin swelling, secondary infection will have redness, swelling, pain and tenderness. There are many reasons why a bag on the back hurts when you touch it, so we recommend that patients seek medical attention and undergo relevant examinations to clarify the cause of the disease.