All of the conditions show muscle degeneration, which causes progressive weakness.
Symptoms include difficulties with walking and climbing stairs, falling, and arm weakness. Swallowing and breathing can also be affected.
Inflammatory myopathies are potentially life-threatening, especially dermatomyositis which can affect the heart muscle and the lungs. In up to 20 percent of cases, dermatomyositis can also be associated with cancer.
Treatments are available for some forms of myositis such as dermatomyositis and polymyositis. About half of the people treated for these conditions recover enough to be able to do the things they did before they became ill.
About 5,000 to 6,000 people in the UK have a form of myositis.