Polymyositis is a type of inflammatory myopathy, which refers to a group of muscle diseases characterized by chronic muscle inflammation and weakness. The muscles affected by Polymyositis are the skeletal muscles (those involved with making movements) on both sides of the body. The disease is more common among women and among black individuals. The exact cause of Polymyositis is unknown. The disease shares many characteristics with autoimmune disorders, which occur when the immune system mistakenly attacks healthy body tissues. In some cases, the disease may be associated with viral infections, connective tissue disorders, or an increased risk for malignancies (cancer). Diagnosis is based on a clinical examination that may include laboratory tests, imaging studies, electromyography, and a muscle biopsy.