2. Stone Man Syndrome
Known medically as Fibrodysplasia ossificans progressiva, this progressive genetic disorder affects up to 2-million people worldwide (which is still pretty rare, when you do the math). The condition turns soft tissues into bone over time, hence the nickname for this disease.
IFLScience.com explains a defect in the ACVR1 gene is responsible—one of its primary functions is to convert cartilage into bone as children grow, but a mutation can cause muscles to convert to bone, and also fuse joints together. There are currently no treatments or cures (surgery to remove bony areas can make it worse, notes the source).