Redness and swelling behind the nail cap may be associated with trauma, onychomycosis, and purulent dactylitis. 1. Trauma: If the nail cap has been hit by collision, extrusion and other external blows, it may cause localized injuries, which may lead to the symptoms of redness and swelling behind the nail cap. 2. Nail infection: the cause of its formation may be related to bacterial infection, allergens and other factors, mainly manifested as redness, swelling, heat, pain and other symptoms of the tissue around the nail. 3. Purulent dactylitis: its cause is usually related to trauma to the end of the finger, inguinitis, and bacterial infection, etc. The patient’s end of the finger will appear red, swollen, and pain, chills and fever and other symptoms. The cause of redness and swelling behind the fingernail cap is not the same, and patients are advised to seek medical examination to clarify the cause of the disease.