The exact cause of ankylosing spondylitis (AS) remains unknown. However, genetics may be a factor, because a gene called HLA-B27 occurs in more than 90 percent of patients with the disease, according to the Arthritis Foundation. However, not everybody with HLA-B27 develops ankylosing spondylitis. People with HLA-B27 who have no relative with AS have only a 2 percent chance of developing the disease, whereas people with the gene who do have a parent or sibling with AS have a 20 percent chance of developing AS.

In 2007 two other gene variants were linked to AS: IL23R and ARTS1. People who have both of these and HLA-B27 may have a 25 percent chance of developing AS.

AS is believed to be an autoimmune disease, meaning that the patient's immune system is hyperactive and mistakenly attacks the body's own tissues. Other examples of autoimmune diseases include rheumatoid arthritis, psoriatic arthritis and inflammatory bowel disease.

AS primarily affects boys and men between the ages of 16 and 35. It is less common in women, although it may develop during pregnancy. It is also normally less severe in women, making it harder to diagnose. AS is more common in American Indians than in other racial and ethnic groups, according to the American College of Rheumatology.

About 5 percent of cases develop in children. AS is more likely to appear in boys than girls. Symptoms in children usually begin in the hips, knees, heels or big toes before progressing to the spine.