Polymyositis is a type of muscle disease called an inflammatory myopathy. It inflames your muscles and their related tissues, like the blood vessels that supply them. It can cause muscle weakness and pain, often on both sides of your body. The common symptoms associated with polymyositis are muscle pain and stiffness, muscle weakness, joint pain and stiffness, difficulty in swallowing etc.

This condition may occur when immune system cells infiltrate and attach muscle tissue, though the cause of polymyositis is indicative of hereditary disease. Current research suggests that the condition may occur when immune system cells infiltrate and attack muscle tissue (an autoimmune process).