Juvenile dermatomyositis (JDM) is a disease in children that causes skin rash (dermato) and muscle inflammation (myositis), resulting in weak muscles. 

JDM is a type of autoimmune disease. The immune system is a group of cells that protect the body from infections. In autoimmune diseases such as JDM, these cells fight the body's own tissues and cells, causing inflammation and, in some cases, tissue damage.  

JDM is different from adult dermatomyositis. In JDM, there is no increased risk of cancer. However, there is often blood vessel involvement, which can be severe. Also, calcium deposits (calcinosis) are common, especially in the recovery phase.