A 5-year-old girl with a serious joint disease called systemic juvenile idiopathic arthritis developed a dangerous fungal infection in her nasal cavity and brain area. The fungal infection, caused by a mold organism called Lichtheimia, occurred because her immune system was severely weakened by medications used to treat her arthritis. Doctors treated the infection with antifungal drugs and surgery, and the patient eventually recovered by switching to a different arthritis medication. This case highlights how the strong medications needed to control some childhood arthritis can unexpectedly cause serious fungal infections.