Mycobacterium murinum infection is a skin disease caused by the invasion of mycobacterium murinum from human skin when humans come into contact with seawater or fresh water. The pathogenesis is still unclear. It is more common in children and young people. The skin lesions are prone to trauma on the elbows, knees, hands, feet, toes, ankles, calves and other parts of the limbs that are susceptible to trauma. Elbow lesions are the most common, accounting for more than 70%. Small, mostly single red papules appear within 3 to 4 weeks of local injury, slowly growing to form 1 to 2 cm red or purple nodules, and very few may rupture to form superficial ulcers. Anti-tuberculosis drugs are ineffective. It usually disappears on its own within 2 to 3 years.