Myositis (Polymyositis / Dermatomyositis) Make an Appointment Find a Doctor Ask a Question Reviewed by Liudmila Kastsianok, MD, RhMSUS (October 01, 2018) Myositis is a disease characterized by inflammation of the muscles. It is often associated with severe muscle weakness. Myositis can also affect other organ systems including the skin, joints, lungs, heart, and gastrointestinal system. It is a chronic disease and can last a long time. The most common forms of myositis are polymyositis and dermatomyositis. Myositis is a systemic autoimmune disease. This means that the body's immune system does not behave normally. Instead of only fighting infections such as bacteria and viruses, the body's own immune system attacks itself. With myositis, the immune system may attack specific muscles resulting in muscle damage and destruction. The immune system may also attack other organs such as the lungs, skin, and joints. Programs & Services Early Rheumatoid Arthritis Clinical Trials For more than 100 years, National Jewish Health has been committed to finding new treatments and cures for diseases. Search our clinical trials.