Child has unexplained fever? My 4yr old daughter has been getting unexplained high fevers (104-105) for more than 1, 1/2 year now. A year ago we took her to the ER while she had a high fever and they kept her in the hospital for 12 days and ran many tests, multiple blood test, lymph node biopsy, bone marrow biopsy just to name a few. They could not find any cause for the fever. She also suffers from extreme joint pain and rashes when she gets the fever. They put her on prednisone (steroid) a yr ago at a high dose and the symptoms got better right away. We were told that she has FUO (Fever of unknown origin)which might be caused by a hyperactive immune system. The prednisone was supposed to control her immune system and bring it down to behaving normally. Since about 3 months ago they started reducing her dose and all of sudden all the symptoms are back. They increased the dose slightly and seems to help the symptoms.  The issue is that she cannot stay this medication for long as it has many permanent side effects.

Summarized question: What are possible causes of unexplained high fevers in children?