A fungus ball occurs when there is an overgrowth of fungal elements in the sinuses, particularly the maxillary and the sphenoid sinuses, where the fungus thrives in the warmth and humidity. Common molds such as Aspergillus are usually responsible. Sometimes, bacteria can also cause infection in the sinus affected by the fungus ball. Typically, only a single sinus is involved, and treatment involves removal of the fungus ball through endoscopic sinus surgery. Most patients have excellent results from surgery, and may not require any further treatment.