Ankylosing spondylitis (AS) is a type of arthritis that affects the spine. AS often involves redness, heat, swelling, and pain in the spine or in the joint where the bottom of the spine (sacrum) joins the pelvic bone (ilium)."
The cause of ankylosing spondylitis (AS) is unknown. It’s likely that genes (passed from parents to children) and the environment both play a role. The main gene associated with the risk for AS is called HLA-B27. Having the gene doesn’t mean you will get AS. Fewer than 1 of 20 people with HLA-B27 gets AS. Scientists recently discovered two more genes (IL23R and ERAP1) that, along with HLA-B27, make it more likely you will get AS."
Source : US NIH
Not only is the cause unknown, but the figures for the number of people worldwide affected are disputed - estimates ranging from 0.1% to 1.8%.
Current opinion is that it's probably due to a combination of unidentified genetic and environmental factors.
Further reading Oxford American Rheumatology Library
Also see : DISHplugin-autotooltip__plain plugin-autotooltip_bigDISH
Diffuse Idiopathic Skeletal Hyperostosis, or DISH (a.k.a. Forestier's disease) was first described over 100 years ago, It affects from 2.5% to 10% of people over the age of 70.
It causes inflammation, and bone overgrowth, at the sites where tendons are attached to bones. It typically affects the spinal vertebrae, but can also affect any other bone, e.g. the neck, shoulders, ribs, hips, pelvis, knees, ankles, and hands. where it leads to stiffness and painful movements.