An umbrella term for diseases which have chronic muscle inflammation and weakness of unknown etiology. The types of idiopathic inflammatory myopathy are further defined by either clinicopathologic criteria or by the presence of certain autoantibodies. The disease is acquired idiopathic inflammatory myopathy (Monarch Disease Ontology term MONDO_0020122). Also known as: IIm, IMM, idiopathic inflammatory myopathies, idiopathic inflammatory myositis.