Lupus, or systemic lupus erythematosus (SLE), is an autoimmune disorder that can range from mild to severe. It's unclear how many people that lupus affects. The Lupus Foundation of America estimates that as many as 1.5 million Americans have it. Most of them are women. Find out more about this autoimmune disease by taking the following quiz, based on information from the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS).