Most people with adult Still's disease have a combination of the following signs and symptoms:

• Fever. You might have a daily fever of at least 102 F (38.9 C) for a week or longer. The fever usually peaks in the late afternoon or early evening. You might have two fever spikes daily, with your temperature returning to normal in between.
• Rash. A salmon-pink rash might come and go with the fever. The rash usually appears on your trunk, arms or legs.
• Sore throat. This is one of the first symptoms of adult Still's disease. The lymph nodes in your neck might be swollen and tender.
• Achy and swollen joints. Your joints — especially your knees and wrists — might be stiff, painful and inflamed. Ankles, elbows, hands and shoulders might also ache. The joint discomfort usually lasts at least two weeks.
• Muscle pain. Muscular pain usually ebbs and flows with the fever, but the pain can be severe enough to disrupt your daily activities.

The signs and symptoms of this disorder can mimic those of other conditions, including lupus and a type of cancer called lymphoma.

It's not certain what causes adult Still's disease. Some researchers suspect the condition might be triggered by a viral or bacterial infection.

Age is the main risk factor for adult Still's disease, with incidence peaking twice: once from 15 to 25 years and again from 36 to 46 years. Males and females are equally at risk.

Most complications from adult Still's disease arise from chronic inflammation of organs and joints.

• Joint destruction. Chronic inflammation can damage your joints. The most commonly involved joints are your knees and wrists. Your neck, foot, finger and hip joints also may be affected, but much less frequently.
• Inflammation of your heart. Adult Still's disease can lead to an inflammation of the saclike covering of your heart (pericarditis) or of the muscular portion of your heart (myocarditis).
• Excess fluid around your lungs. Inflammation may cause fluid to build up around your lungs, which can make it hard to breathe deeply.
• Macrophage activation syndrome. This rare, but potentially fatal complication of adult Still's disease, can cause low blood cell counts, very high triglyceride levels and abnormal liver function.