Still's disease is one type of juvenile idiopathic arthritis (JIA) and is also known as systemic-onset JIA. It is referred to as "juvenile" because it typically affects children under 16 years of age. Adult Still’s disease is a rare type of inflammatory arthritis that is similar to rheumatoid arthritis. It shares characteristics of systemic-onset juvenile idiopathic arthritis, but it begins in adulthood. Although some features are similar, adult-onset Still's disease is different than Still's in children. In children, Still's disease is considered a form of juvenile rheumatoid arthritis and referred to as systemic-onset juvenile rheumatoid arthritis. Less than one in , people develop adult-onset Still's each year and it is more common in women.