A 65-year-old woman who received a lung transplant developed a rare and serious brain infection caused by an unusual fungus called Rasamsonia argillacea. The infection was difficult to diagnose because it looks similar to more common molds under the microscope. During the investigation of this unexpected infection, doctors discovered that the patient had a rare genetic disorder affecting her immune system that had never been diagnosed. This case highlights how unusual infections can reveal hidden immune system problems.