http://en.wikipedia.org/wiki/Fibrodysplasia_ossificans_progressiva Fibrodysplasia ossificans profressiva is a (mostly) genetic disease that turns all muscles, tendons, ect. into bone. Mostly takes affect by the age of 10, but can take place at any time in life. 1 in 2 million people on Earth are affected by it.
Doctors attempting to remove the new "bone" results in it coming back faster and stronger. There is no treatment, and no cure. It continues until the person is encased and trapped by their own skeleton. Unable to move anything at all but their lips. (No, can't even open their mouth). The person usually has to choose a position they would like to spend the rest of their life in, before it completely immobilizes them.
Now wouldn't it be nice if someone made a Creepypasta about it?