“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%
Further reading Oxford American Rheumatology Library
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