This forum is for "Myositis" (not "mytosis"). The word myositis comes from "Myo-" meaning muscle, and "-itis" meaning inflammation of. In general terms, Myositis is inflammation of muscle tissue.
Specifically, there is a family of auto-immune disorders that cause muscle inflammation called Myositis. These include DM - Dermatomyositis (muscle inflammation with a skin rash; the rash is actually caused by the exact same process that causes the muscle problems), PM - Polymyositis (think of it as DM without the rash), IBM - Inclusion Body Myositis (the muscle inflammaion is caused by amyloid deposits, similar to those found in the brains of Alzheimer's patients), and JDMS - Juvenile Dermatomyositis (DM diagnosed in kids under 18).
I hope that helps explain it to you.