Myositis is an inflammation of the muscles, which can be caused by injury, infection, reaction from certain medications, and even exercise. Some forms of Myositis are chronic and idiopathic, meaning the cause is unknown or not well understood. Types of Myositis include dermatomyositis (DM), polymyositis (PM), juvenile myositis (JM), and inclusion-body myositis (IBM).
Symptoms of myositis can include muscle weakness, muscle pain and muscle tenderness. Other symptoms vary, depending on the specific cause of myositis.