Now the Imagine Dragons frontman is opening up about a medical condition he has. It's called ankylosing spondylitis (abbreviated to 'AS'), and it's an autoimmune disease that affects the spine. Similar to arthritis, this ailment causes debilitating back pain. Reynolds said four people in his family have also been diadnosed with AS.He's sharing the news to help spread awareness. He asks that people take a quick three-minute quiz to see if they could potentially have AS as well.