Myositis -A general term for inflammation of the muscles. Many such conditions are
considered likely to be caused by autoimmune conditions, rather than directly due
to infection (although autoimmune conditions can be activated or exacerbated by infections).
The diseases dermatomyositis and polymyositis both involve myositis. Polymyositis
causes muscle weakness, usually in the muscles closest to the trunk of your body.
Dermatomyositis causes muscle weakness, plus a skin rash.
Polymyositis and dermatomyositis are a mixture of various inflammatory diseases of
the muscles with or without inflammation of skin.
Both polymyositis and dermatomyositis have an autoimmune basis.
