Causes Make an Appointment Refer a Patient Ask a Question Reviewed by Liudmila Kastsianok, MD, RhMSUS (October 01, 2018) Who Gets Myositis? Although myositis is a rare disease, people of all races and ethnic backgrounds get the disease. The peak age of onset is in the 50s, although it can occur at any age. Inclusion body myositis is more common in men, while dermatomyositis and polymyositis are more common in women. What Causes Myositis? The cause of myositis remains unknown. We do know that myositis is an autoimmune disease. In myositis, autoimmunity leads to inflammation of specific muscles. This inflammation results in muscle damage and may lead to muscle cell death. Many different muscle groups may be involved in the disease ranging from muscles of the arms and legs to those of the respiratory and gastrointestinal systems. In addition, the abnormal immune response can lead to other problems including, damage to blood vessels, joints, lungs and the heart. Symptoms 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.