What is a black line under your fingernails?

If the black line under the nail is on the nail deck, it is considered that the patient's nail has been hit, or the nail deck has been pierced during the nail removal process, resulting in nail bed bleeding symptoms. In addition, it may also be caused by fungal infection, commonly known as onychomycosis. Fungi eat away at the patient's nail, making the nail color gradually black, often accompanied by brittle nail, easy to fall off symptoms. This disease needs timely treatment, can choose some external antifungal ointment, pay attention to keep a health.