D2I2.
autoimmune

Polymyositis

Polymyositis (PM) is a type of of the muscles related to dermatomyositis and inclusion body myositis. Its name is derived from poly- 'many'; myos- 'muscle' and -itis 'inflammation'. The inflammation of polymyositis is mainly found in the endomysial layer of muscle, whereas dermatomyositis is characterized primarily by inflammation of the perimysial layer of skeletal muscles.

Underlined words are explained — tap any of them.

Plain-language summary adapted from Wikipedia. Not medical advice.