Natural History, Genetics, and Pathophysiology of Systemic Juvenile Idiopathic Arthritis, Adult-Onset Still's Disease, and Related Conditions

Background: Inflammatory conditions can cause symptoms like fevers, arthritis, and rash. Systemic juvenile idiopathic arthritis (sJIA) is one of these conditions. So is adult-onset Still s disease (AOSD). Their causes are unknown. Researchers want to learn more about these conditions. This includes genetic changes and environmental factors. Objective: To study sJIA and AOSD in children and adults over time. Eligibility: People with known or suspected sJIA, AOSD, or similar inflammatory condition Design: Participants will be screened with a phone call. Participants will have 1 visit. It may be outpatient or they may be admitted to the clinic. The visit may last up to 5 days. Participants will have: * Medical history * Physical exam * Musculoskeletal exam * Questions about overall health and quality of life, disease activity, functional status, and cognitive ability. Participants may also have: * Pictures taken of their skin, joints, or spine * Blood, urine, and stool tests * Scans or X-rays of joints with arthritis * Chest X-ray * Heart tests * Skin biopsy. The skin will be numbed. The top layers of a small area will be scraped off. Participants who have a joint aspiration may provide a fluid sample. The joint will be prepared, then fluid is removed by needle. A corticosteroid may be injected. Participants who have a bone marrow biopsy may provide sample cells. Participants may be seen by NIH specialists. Members of the participant s family and healthy volunteers may give blood or saliva samples for genetic testing. Participants may repeat some study tests every 6 months.