Polymyositis / Dermatomyositis is autoimmune disease with Inflammatory directed to muscle. Cause is unknown and can affect individuals of all ages. The annual incidence of polymyositis and dermatomyositis ranges from 5–10 new cases per million. The average female-to-male ratio exceeds 2:1, with women predominating in this disease that occurs characteristically between ages 15 and 44 years.
The idiopathic inflammatory myopathies are immune-mediated processes, believed to be triggered by environmental factors in genetically susceptible individuals. The specific causes or triggering events remain unknown, but viruses have been strongly implicated.