<s>[INST] <<SYS>> This is a medical question of type : frequency <</SYS>> How many people are affected by ankylosing spondylitis ? [/INST] Ankylosing spondylitis is part of a group of related diseases known as spondyloarthropathies. In the United States, spondyloarthropathies affect 3.5 to 13 per 1,000 people. </s>